US2237391A - Apparatus for the control of highway crossing signals - Google Patents

Description

A N 8, 1941. w. D. SELLERS APPARATUS FOR THE CONTROL OF I IIGHWAY CROSSING SIGNALS Filed April '7, 1939 2 Sheets-Sheet l MWWQTI 1941' W. D. SELLERS APPARATUS FOR THE CONTROL OF HIGHWAY CROSSING SIGNALS Filed April 7, 1939 2 Sheets-Sheet 2 Patented Apr. 8, 1941 UNITED APPARATUS FOR THE CONTROL OF HIGH- WAY CROSSING SIGNALS William D. Sellers, Glen Ellyn, Ill., assignor to Western Railroad Supply Company, Chicago, 111., a, corporation of Illinois Application April '7, 1939, Serial No. 266,548

(Cl. MEL-130) Claims. I

The present invention relates to railroad-highway grade crossing warning signals in general and particularly to a signal controlling system in which the signal is operated for a predetermined time interval before the arrival of the train at.

the crossing regardless of the speed of the train. More specifically the invention comprises a railroad-highway grade crossing warning signal control system inwhich the speed of the train through a relatively short track section causes the control mechanism to so operate that the warning signals are placed into operation with the train at varying distances from the railroadhighway intersection, depending upon the speed of the train, in order that the time interval which will have elapsed between the time of signal operation and train arrival will be a fixed period regardless of the speed of the train.

It is an object of the present invention to provide a new and improved train actuated highwayrailroad grade crossing signal control system. Another object of the invention is to provide a crossing signal operating system for railroads in which the speed of a train through a short track section determines the position on the track at which the crossing signal is placed into operation. Another object of the invention is to provide a railroad-highway signal-controlling mechanism in which a constant speed electric motor actuates the control mechanism during the passage of the train through the relatively short track section and in which the run down period is in proportion to the actuating period in accordance with a predetermined relationship. Still another object of the invention is to provide a signal-controlling mechanism in which a constant speed motor rotates a weight-lifting cam during the traversal by the train of a relatively short track section, the cam being so designed as to lift the weights at varying distance depending upon the time of the traversal. A still further object of the invention is to provide a railroad-highway grade crossing signal control system in which a weight element is lifted at a varying rate during the time interval required by the train to traverse a short track section and in which the weight falls by gravity at a constant speed thereafter. These and othermore specific objects will appear upon reading the following specification and claims and upon considering in connection therewith the attached drawings to which they relate.

Referring now to the drawings in which a preferred embodiment of the present invention is I disclosed:

Figure l is a diagrammatic illustration of the system comprising the present invention in the track-clear condition;

Figure 2 is a diagrammatic illustration of the electrical circuit only of the system;

Figure 3 is a top view of the finger-positioning solenoid-operated ring;

Figure 4 is an enlarged view showing the connection between the electrical solenoids and the finger-positioning mechanism.

The automatic operation of highway-railroad grade crossing signals is quite old but recently the increase in train speeds has presented new problems in signal control not heretofore confronted. With trains today traveling at speeds varying up to miles per hour, it has become necessary to provide highway-railroad intersection signals having operating characteristics designed to accommodate these high speed trains. in the past with train speeds relatively uniform and lower, it was possible to have the intersection signal operated when the train passed a fixed point at a distance from the intersection. While this system caused a longer period of signal operation for slow trains than for fast trains the difference was not so great as to render the period objectionable to the public for the slow trains. It is recognized, of course, that the intersection signals must be operated for at least a minimum period before the arrival of the train at the intersection and this period has by common acceptance been set at approximately twenty seconds. If, under the old system, the setup is such that the signal is operated for twenty seconds before the arrival of the fast trains at the crossing, then the period of operation prior to the arrival of the slow trains will be greater than twenty seconds in inverse ratio to the speed of the trains.

In the control system constructed in accordance with the present invention, the intersection signal is operated at a predetermined time interval before the arrival of the train at the highway-railroad intersection regardless of the speed of the train. The warning period can be selected and the mechanism designed in accordance with that period to give the precise operation intended. In the system constructed in accordance with the present invention, the time interval for the train to traverse a relatively short track section gauges the wind-up of the timing mechanism and during this period a constant speed motor causes a rotating cam to lift a weight element at a varying rate in accordance with a predetermined formula so that, at the end of the wind-up period, which is determined by the train reaching the end of the short track section, the weight element can return under gravity to its initial position in a time interval which is so related to the wind-up period that the intersection signal is operated to give the desired warning period.

Referring again to the drawings, and to Figure 1 in particular, a preferred embodiment of the invention is illustrated. A railroad track is seen to approach a highway and to be divided into mutually insulated sections AB, BC, and CD; the highway crossing the track adjacent the point D. Each track section is energized by its own battery, indicated at I, 2 and 3, and in parallel with the battery and across the rails of each section is a relay coil, indicated at 4, and 6. The relays 4, 5 and 6 are normally energized by the batteries I, 2 and 3, respectively, and are adapted to be shunted by the entrance of a train into the track section to which they are connected.

Relay 4 in section AB is provided with a movable contact 4A and with a stationary contact 4B. With the track clear and the relay 4 energized 4A is spaced from contact 43. Upon the entrance of a train into section A--B, however, the relay is shunted and contact 4A dropped into seating relationship with contact 4B pair of movable contacts 5A and 5C which are normally held in seating relationship with stationary contacts 5B and 5D respectively, with the relay energized. With a train present insection BC the relay is shunted and the con tacts 5A and 5C moved to open position. In section CD, relay 6 has a single movable contact 6A which is normally held in seating relationship with stationary contact 63, the contacts open ing upon the relay being shunted by the entrance of a train into section CD.

Relay I, the current to which all the remainder of the system is intended to control, is provided with a single movable contact IA and a stationary" contact 13 which are connected in series with and control the flow of current to the highway signal 8.

The timing mechanism comprises a rotatable, cylindrical cam I2 which is rotatable upon a vertical shaft I3 carried by bearings I4 and which is formed upon its exterior with a camway I5 the vertical displacement of which is related to the circumferential displacement in accordance with the formula weight or mass I6 is of suflicient diameter to permit the weight to move longitudinally of the cam I2 in spaced relationship thereto. A plurality of cam-contacting fingers 29 are pivotally Relay 5 of section BC is provided with a mounted upon the weight I6 and are adapted in 7 their lowermost positions to contact the camway I5 to ride thereon to lift the weight vertically upon the rotation of the cam. Only one of the fingers 20 will actually ride upon the camway I5 in the operation of the mechanism although a multiplicity of such fingers are provided around the cam I2 to insure that vertical displacement of the weight will begin immediately upon rotation of the cam.

Cam-contacting fingers 20 are adapted to be held in their lowermost positions by an encircling ring element 22 which overlies the top of the inner end of the fingers and which is adapted to be drawn downwardly by pins 23 slidingly mounted in the weight I6. Pins 23 are connected through a pivoted lever arm 24 to the armature 25 of a solenoid 26. The weight of the armature 25 exceeds that of the ring 22 and pins 23 so that when the solenoid is unenergized the fingers 20 are permitted to pivot upwardly and from the path of the camway I5. With the solenoid energized, however, the ring 22 is pulled downwardly, the armature 25 moving upwardly in the solenoid 26 and one of the pivot elements 20 is pressed into contact with the camway I5 of cam I2 so that upon rotation of the cam the weight is lifted vertically at a variable rate determined by the cam slope. Abutting shoulders at the inner ends of the fingers 20 which are formed on the Weight I6 determine the lowermost position of the fingers.

A driving motor M is provided which drives a worm 29 which meshes with a worm gear 30 upon a shaft 3| aligned with the first shalt I3. Shaft 3I is adapted to be clutched to cam-carrying shaft I3 by means of a magnetic clutch comprising the slidable disc element 34 mounted upon the shaft 3! which is adapted to be moved magnetically by a stationary electromagnet 35 into contact with a second clutch plate 36 carried by the shaft I3. With the magnet 35 energized the discs 34 and 36 are held together and the shaft I3 is clutched to the driving shaft 3I.

A battery B is connected to the motor M and to the winding of magnet 35 and solenoids 26 through circuits to be described and provides the actuating power for the mechanism.

The weight I6 is adapted to be lifted upon the rotation of the cam I2 at a speed determined by the contour of the camway I5, the slope of which is not constant but instead varies as hereinafter explained. It is clear that the weight cannot be raised at a constant rate if the height to which it is raised is to determine the time delay before a highway signal is placed into operation. Using such a constant rate a constant warning period could not be obtained, assuming a constant rundown speed, but instead the only result would be to maintain a fixed ratio between the length of the warning period and the period of time required for the train to traverse the operating section. The weight is adapted to fall at a constant speed once the locking fingers 20 are released magnetically. To insure the constant speed of fall, a rope, cable, chain or other suitable flexible connecting means 39 is provided which is secured to the weight I6 and which extends upwardly therefrom to pass over a fixedly mounted pulley 40, and over a second pulley 4|, formed with escapement teeth, on the far side of which it carries a weight 42 of sufiicient mass to maintain the cable or rope 39 taut. A pivoted escapement element 44 cooperates with the teeth of pulley M and functions, with the weight I6 released from the cam, to cause which will hereinafter become apparent.

the weight to fall at a constant speed which is predetermined.

In its vertical movement the weight i6 functions to operate certain contacts for reasons erally extending abutments 4G and 41 are formed with insulated outer ends and contact respectively tiiBli'lOl/Elblii contact elements 48A and 49A of switches and M3. The initial upward movement of the weight causes abutment element 45 to move from contact 48A and that element under its normal spring bias seats upon the stationary contact element 483 to close switch 48. The travel of the weight IE to its upper limit brings the abutment 41 into contact with the movable element 49A of switch 49 and causes that element to move from the stationary contact 493. Switch lil is normally a shunt around a. high resistance holding coil 50 which is in series with the motor M, and the opening of the switch reduces the current to the motor to the point such that it cannot continue to lift the weight I6 nor do more than hold it in a. fixed position.

Referring now particularly to Fig. 2 the electrical circuit of the system is clearly disclosed.

it is seen that in the circuit of the motor M and the battery B are the contacts 3A, 4B and 5A,

and. a parallel pathway through the weightcontrolled contacts WA and 4913 with the hold coil As switch 4 is normally open and contact lA spaced from contact tB it is seen that the closing of these contacts, which occurs on the iunting of relay t coincident with the entrance of a train into section A--B, completes the motor circuit. It is also seen that the entrance of a. train into the circuit B-C will shunt the relay ti and cause contact 5A to move from contact 513 which will open the circuit. Also it is seen that the opening or" contacts 49A, 1913, which is accomplished by the weight-carried abutment 41 at the upper end of the weight travel does not open the motor circuit but merely places the high reance hold coil 50 in series therewith.

Figure 2 also makes clear that the windings of the magnet 35 and of the solenoids 26 are in parallel with the motor and that current will flow therethrough so long as current flows through the motor.

In the circuit with the winding of relay 1, which directly controls signal operation l5 seen to be the contacts 5A, 6B of relay 6, together with a parallel pathway through the contacts 5C, SD of relay and MA, MB of weight-controlled switch til. The battery B is also in this circuit. Upon the initial movement of the weight It contacts sea, WE are closed, thereby placing a shunt around contact EC, 5D of relay 5, thereby preventing the cutting off of the current from the relay l immediately upon the entrance of a Win and the section B-C. The weight must rot .11 to its lower position so that its abutment will function to open switch 48 before current can he out ch from relay 1. Contact 6A and LB of ii, which is positioned in section C--D, are effective to open the circuit through relay 1 only in the event that failure has occurred and the train has entered section C-D before switch has been opened by the weight.

The operation of the system constructed in ac- 106 with the present invention is as follows, re; use being had to the various figures of the drawings, in which, it is to be remembered, the condition illustrated is the track-clear condition.

"the condition of the system will now be described Lati with the various positions of the train along the track.

Track-clear condition Train enters section AB A train enters initial and short track section AB moving in the direction of the highway crossing. Relay 4 is, of course, shunted and deenergized and contact 4A drops into contact with stationary contact 43. Reference should be had particularly to Figure 2 in which the electrical circuit is more clearly illustrated. A circuit is thereupon completed through the motor M and battery B and which includes the contacts 5A and 3B of relay ii and 49A and 49B of switch 49. The motor M begins to rotate instantaneously upon the entrance of the train into section A--B. Simultaneously with the energization. of motor M is the passage of current through the winding of magnet 35 and the solenoids 26. The former effects closure of discs 34 and 35 and the clutching of the shafts 3| and I3 while the latter causes the downward movement of the finger-contacting ring 22 and the forcing into contact with the camway [5 the nearest adjacent pivoted finger 20, in advance of the camway IS.

The rotation of the motor M is seen, therefore, to produce instantaneous rotation of the cam l2 which thereupon lifts the weight It vertically in which direction it is guided by the fixed guide rods I8. This rotation of the cam l2 and the vertical displacement of the motor continues only so long as the train is in the section A-B and until it enters the section B-C and results in the vertical displacement of the weight IS in accordance with the formula and 5D of relay 5 which will be opened immediately upon the train entering section B-C. Current will therefore continue to flow to the signal-controlling relay '1 through the switch contacts 48A and 1813 after the train has entered section B-C which would not be the condition were it not for the switch 48 being closed.

Normally the period of time during which the train is positioned in section A--B only will cause the weight to be lifted to some intermediate position within the limits of the cam I2. Should the train stop within section AB, however, or be so excessively slow as to take a. time period sufiicient for the weight it to be lifted the en tire height of cam l2 the abutment element 41 of the weight causes the movable contact 49A to move from the stationary contact 493 of the switch 29 thereby removing the shunt around the high resistance hold coil 53 and placing that ele- I2 should continue to rotate.

Train enters section B-C The front of the train passes from section AB into section B-C. Relay 5 is de-energized and contacts 5A and 50 move from the stationary contacts 513 and 5D respectively. The current through the motor and through the magnet 35 and the solenoids 26 is immediately broken, as is evident in Figure 2, upon the opening of the contacts 5A and 5B. The opening of contacts 50 and 5D, however, does not affect the flow of current through the signal-controlling relay 1 as the weight-controlled contacts 48A and 48B are shunted therearound.

The de-energization of the solenoids 23 causes the ring 22 to move upwardly and release the cam-contacting finger'zt so that the finger pivots upwardly whereupon the weight I6 immediately begins to fall. The speed of downward movement of the weight .IB under the force of gravity is controlled by the escapement 44 which acts upon the escapement wheel 4! which carries the connecting rope or cable 39 and which has been maintained at all times in a taut relationship by the weight 42 at its opposite end. Friction between the pulley 4| and the cable is such that movement of the rope 39 is permitted only upon the rotation of the pulley 4|. Under the control of the escapement 44 the weight l6 advances downwardly at a constant speed requiring a time interval which is directly proportional to the distance which it has been lifted by the cam I2. The time for the weight to return to its initial position will be related to the time required for it to be lifted to its final'position by the cam [2 directly in accordance with the formula above set forth, Tex being taken as the run-down time and TAB being taken as the wind-up time. If the speed of downward movement of the weight [6 is exactly equal to the peripheral speed of the cam I! then the distance which the weight is lifted is to the peripheral distance traveled by a point on the cam as Tex is to TAB At the end of the rundown period Tex weight I 6 will have moved under the actuation of gravity down to its lowermost position and the abutment 46 carried thereby will have actuated the movable contact 48A of switch 48 and will have moved it from the stationary contact 48B. As contacts 5A and 5C were opened upon the shunting of relay 5, current is thereupon out off from the signal-controlling relay 1 which is in series with the switch 48 and the contacts 50 and 5D and the battery B. Relay 1 being open, the movable contact 1A moves into contact with the stationary contact 1B and current flows to the signal 8 which is thereupon operated.

This operation and opening of contacts 48A and 48B took place at a time TBx after the train entered section BC. Referring to Figure 1 in particular the train would be at some point or position X between the points B and C at the time the switches operated which point X would be spaced at a. distance from the point D such that it would require the train a time period K,

equal to the desired warning period, for the train to reach the highway from X.

In accordance with the present invention the signal is always placed in operation when the train is at such a variable point X from. the point D that the desired Warning time interval K will have elapsed before the train reaches the point Dand the highway. This result is obtained, as has been described, by lifting the weight IS a distance such that it will require a time interval TBx, in which time interval the train will have traveled from B to point X, to return to its initial position.

The formula which is used is arrived at by the following reasoning:

Regardless of the train speed it is desired that the signal begin its operation a predetermined period before the train reaches the highway. This warning period is a constant which may be called K and can be taken, for example, to be 20 seconds. Referring to Figure 1 the dotted line X-X is a variable position on the track between B and D at which the train is positioned when the signal begins operation and from which it will require twenty seconds for the train to reach the crossing. Let Tex be the time required to travel the distance B to X for any particular train. Obviously for a fast train X will be farther from D than for a slow train for both are to reach the crossing during the same elapsed period from position X. It is clear that if the signal can be put into operation for each particular train when it reaches its X position that the desired result will accrue. It has been calculated that for any given train Where the constant K is decided upon and the distances A-B and B D known, it is clear that Tex can be calculated by substituting in the formula the value of TAB. Of course TAB is the time required by the train to travel the distance A-B, etc. As the range of train speeds is known the calculation can be done by assuming values of TAB. It is of course obvious that the proper units must be used, for example, K, TAB and Tex in seconds and BD and AB in feet.

The formula is derived in the following manner:

TAB-AB TBD E and B (2) Thaw- 2 also BX= BDK from which BD= BX substituting in (2) the Value of Ten in (4) gives BD nX TAB'E B1) TEX: TAB'E K It is of course within the scope of the invention to vary the rate of descent of the weight l6 relative to the peripheral speed of the cam, It may be found more desirable to have the Weight descend at one-fifth or one-tenth the cam peripheral speed, in which event the vertical displacement of the cam would be altered by the same factor.

I claim:

1. In a control system for railroad-highway grade crossing signals, a plurality of mutually insulated track sections including a first section and a second section approaching a highway, a signal at the highway, a norm-ally energized relay in each of said sections adapted to be shunted by the entrance of a train into the section in which it is connected, and means responsive to the passage of a train through said sections in the direction of the highway to cause the operation of the signal at a predetermined time interval prior to the arrival of the train at the highway; said means comprising a constant speed motor energized by the operation of the relay in the first track section, a vertically displaceable weight, a cam driven by said. motor to lift said weight at a variable rate, means to releasably connect said weight to said cam for vertical displacement thereby during the actuation of said cam by said motor, said releasable connecting means being controlled by the relay in the second track section adjacent said first section, speed controlling means governing the uniform rate of descent of said weight, and means operated by said weight by its downward movement to control said signal.

2. In a control system for highway-railroad grade crossing signals, a plurality of mutually insulated track sections approaching a highway and including a first section and a second section, a signal at the highway, a normally energized relay in each of said sections adapted to be shunted by the entrance of a train into the 520- tion in which it is connected, and means responsive to the passage of a train through said sections in the direction of the highway to cause the operation of the signal at a predetermined time interval prior to the arrival of the train at the highway; said means including a vertically displaceable weight, a movable cam to displace said weight at a variable rate, a motor to actuate said cam, electrically actuated connecting means to connect and disconnect said weight to and from said cam, the motor and connecting means being in the electrical circuit of normally open contacts of the relay of the first track section and normally closed contacts of the second and adjacent track section, the shunting of the first track section relay being effective to complete the electrical circuit whereupon said cam is driven by said motor and said weight is lifted thereby and the shunting of the second track section relay being effective to open said circuit whereupon said connecting means are de-enerized and weight is released from said cam to fall by gravity and said motor is de-energized, and means to control said signal operated by said weight in its return travel.

3. In combination, a railway track intersected by a highway, a signal at the intersection, and signal-controlling means to operate the signal at a predetermined time period prior to the arrival of a train at the intersection; said signal-controlling means comprising a weight normally occupying one position, a spiral cam arranged for rotation about a vertical axis to lift said Weight at a variable rate, a constant speed motor to actuate said cam, train-operated means to energize said motor during the time interval. required for a train approaching said highway to traverse a fixed distance, train-controlled means to disconnect said weight and cam at the end of said time interval and to permit said weight to fall to said one position, means to cause said weight to fall at a constant speed and means effective when said weight has fallen to a predetermined position to place said signal into operation.

4. The construction recited in the preceding claim characterized in that said means to disconnect said weight and cam comprises a solenoid operated latch controlled by a relay in a track section adjacent or track section comprising the fixed distance and between the latter and the intersection,

5. In combination with a railway track formed with insulated track sections and intersecting a highway, a signal at the intersection of the track and the highway, and means to operate said signal a fixed time interval prior to the arrival of an approaching train at said intersection; said means comprising a vertically displaceable gravity-biased element, a rotatable cam to lift said element at a varying rate, a motor to drive said cam durin the passage of a train through a first track section, means actuated by said element at a fixed point in its displacement to insert a currentdimiting resistor in the circuit of said motor, means effective upon the train entering a second track section to effect the reversal of direction of travel of said element, speed-controlling means including an escapement to effect a constant speed of return travel of said element, and means effective upon said element reaching one point in its return to place the signal into operation,

6, In combination with a railway track formed with insulated first and second track sections and intersecting a highway, a signal at the intersection of the track and the highway, and means to operate said signal a fixed time interval prior to the arrival of an approaching train at said intersection; said means comprising a movable cam and a vertically movable weight, electrically actuated means to connect said weight to said cam carried by said weight and movable therewith, means effective upon the entrance of a train into the first track section to move said cam and to energize said connecting means whereby said weight is lifted by said cam at a variable rate, means effective upon said train entering said second track section to de-energize said connecting means whereupon said weight is free to fall under the force of gravity, means controlling the speed of fall of said weight at a constant speed, and means effective upon said weight reaching a certain point in its fall to place the signal into operation.

7. In combination with a railway track formed with insulated first and second track sections and intersecting a highway, a signal at the intersection of the track and the highway, and means to operate said signal a fixed time interval prior to the arrival of an approaching train at said intersection; said means comprising a rotatable cam, a constant speed motor, means effective upon the entrance of a train into the first track section to energize said motor and to connect it to said earn, a vertically displaceable weight adapted to be lifted at a variable rate by said cam, means effective upon the entrance of a train into said first track section to associate operatively said weight with said cam whereupon the movement of said cam effects the lifting of said weight, means effective upon the entrance of the train into said second tracksection to de-energize said motor and said associating means whereupon said weight is free to fall under the action of gravity, means to eiTect a constant speed fall of said weight and means effective at a fixed point in the downward movement of said weight to place said signal into operation.

8. In combination with a railway track intersecting a highway and having first and second mutually insulated track sections, an energized relay in each section adapted to be shunted by the presence of a train, a signal at the intersection, and means to operate a signal at the intersection a predetermined time interval prior to the arrival of a train; said means comprising a spiral cam having a variable slope and rotatable about a vertical axis, a driving motor for said cam, a magnetic clutch between said cam and motor, the circuit of said clutch and motor being controlled by the relay of a first track section which is efiective to energize said clutch and motor upon the entrance into said section of a train, a vertically displaceable weight, electrically-energizedconnecting-means to connect said weight to said cam so that the rotation of said cam can efiect the lifting of said weight at a variable rate, said electrically energized connecting means being controlled by the relay of the first track section which effects the energization of said means to connect said weight to said cam with a train in said first track section, the relay in a second track section between said first section and the intersection also controlling the flow of current to said motor, clutch and electrically energized connecting means and adapted to disconnect same electrically upon the entrance into said second track section of the train, whereupon said weight is disconnected from said cam and is free to fall under the force of gravity, means to effect a constant speed in the fall of said weight, and means effective upon the return of said weight to its initial position to place said signal into operation.

9. In combination with a railway track intersecting a highway and including a first section and a second section, a signal at the intersection, and means to operate said signal at a predetermined time period before the arrival of a train at said intersection; said means comprising a vertically displaceable element, means to eifect the vertical displacement of said element at a variable rate during the entire time interval required by a train approaching said intersection to travel said first section of said track, said last mentioned means including a constant speed rotary motor and a cam rotatable about a vertical axis, means to release said element to return by gravity to its initial position upon said train entering said second track section between said first section and the intersection, means actuated by the element to eifect a constant speed return, said cam being contoured to raise said element a distance such that the rundown time for said mechanism is related to the windup time in accordance with the formula length of first track section T (rundown) T (Wmdup) length of second track scctlon where T is time and K is a warning period in the same unit of time.

10. In combination with a railway track intersecting a highway and including a first section and a second section, a signal at the intersection, and means to operate said signal at a predetermined time period before the arrival of a train at said intersection; said means comprising a vertically displaceable element, means to effect the vertical displacement of said element at a variable rate during the entire time interval required by a train approaching said intersection to travel said first section of said track, said last mentioned means including a driving motor and a spiral cam rotatable about a vertical axis and formed at its upper limit as a continuous fiat surface, electrically actuated means to associate said element with said cam during the time interval the front of a train travels in said first section and to disassociate said element from said cam upon said train entering said second track section, said element being adapted to return by gravity to its initial position upon said train entering said second track section between said first section and the intersection, means actuated by the element to efiect a constant speed return, said cam being contoured to raise said element a distance such that the rundown time for said mechanism is related to the windup time in accordance with the formula WILLIAM D. SELLERS.

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