What prevents points changing under trains?

[edcayton]

I am thinking in particular of situations when a train would often be stopped over a set of points (for example when there is a crossover part way along a platform) and the points being controlled by a signalman who can not see the situation from his box.

 

What device is used to prevent the point being changed under the train, with the consequence that on re-starting the back part of the train would take a differemt route from the front?

 

I gather that this occurence was not unheard of on the real railway, but how common was it?

 

Thanks in advance

 

Ed

[mike morley]

Its called a Facing Point Lock.

They came in different forms, but most were minor variations on a theme of a bar set close to the tops of the rails that was depressed by the flanges on any rolling stock in the vicinity and, via an attached linkage, prevented the points being changed.

Another part of the FPL was a linkage to the signal box where another lever had to be pulled to unlock everything before the point lever could be pulled. The FPL lever had to be returned to re-lock everything before anything else could be operated.

Later (possibly still current) variations on the theme use track circuiting as alternatives to the rail-height bar.

[flyingsignalman]

Various methods that I can think off are:-

Facing Point Locks on the points themselves.

Detection or Clearance bars; these stopped the signalman moving the point lever in the frame while the train was there.

Track Circuits; these also stopped the signalman moving the point lever.

The signal box interlocking stopped point levers being moved when signals protecting the points were cleared. Some interlockings also had the points leading up to a signal locked by that signal being cleared too.

[Glorious NSE]

The modern version would be that the point would be included in a track circuit, and the point could not be changed whilst that track circuit is occupied (EDIT - or a route is set through it)

[edcayton]

I kinda knew about facing point locks, but didn't realise they could be operated by wheel flanges. Is it not possible for a long vehicle to "bridge" this device?

Its called a Facing Point Lock.

They came in different forms, but most were minor variations on a theme of a bar set close to the tops of the rails that was depressed by the flanges on any rolling stock in the vicinity and, via an attached linkage, prevented the points being changed.

Another part of the FPL was a linkage to the signal box where another lever had to be pulled to unlock everything before the point lever could be pulled. The FPL lever had to be returned to re-lock everything before anything else could be operated.

[Camberwell Green]

What era of signalling technology are you asking the question in relation to?

 

In "modern" systems the interlocking will not allow the points to be commanded to move if the train detection sections (track circuits or axle counters ) that include the points are occupied. I say "modern" because the track circuit is quite an old technology now. However, before that an alternative method was the fouling bar; this was a device fitted to the inside of the rail in the vicinity of the points which would be depressed by the flanges of the wheels of any vehicles stood over it (like an elongated treadle) and when depressed prevented the points from being moved.

[edcayton]

I just wrote a post about this and deleted it! I'm thinking of a secondary/branch line circa 1960.

 

Ed

[Camberwell Green]

The purpose of the facing point lock is to secure the switch rails of a set of facing points firmly against one or other of the stock rails to prevent the points standing "open" which would cause an approaching train to derail as the stock rails spread apart. It does not, in itself, prove that there is no train standing over the points and therefore it would be quite possible (assuming mechanical signalling for a moment) for the signaller to release the FPL and pull the points under a train if there was no other means of detecting the position of a train.

[mike morley]

Is it not possible for a long vehicle to "bridge" this device?

 

They were tailored to the longest vehicle operated by the railway concerned.

[Guest stuartp]

I kinda knew about facing point locks, but didn't realise they could be operated by wheel flanges. Is it not possible for a long vehicle to "bridge" this device?

 

At one time there was a limit on the distance between bogies on longer vehicles to ensure that at least one wheelset was depressing the bar at any one time. At least one of (I think) the LMS dining cars had an odd appearance as a result as the bogies are set too far in aesthetically speaking.

 

The rules appear to have been less strict on engine release crossovers in terminal platforms. I can think of a few branch terminii where the engine release crossover was not equipped with FPLs, and one (Dalmellington) where the two ends of the crossover were hand points not even co-acting with each other !

[edcayton]

I can understand that. It OUGHT to be bleedin' obvious to anyone operating point levers if there is a train there, but stupid things do happen-witness the video posted on here recently showing the German railcar/steam loco "coming together">

 

Ed

[The Stationmaster]

Simple set of answers -

1. Handworked points (sidings in yards etc) - absolutely nothing except leaving the operating lever adjacent to the point well alone.

 

2. Trailing points in running lines and facing points on Goods etc lines which didn't have a Facing Point Lock (FPL) in the days of mechanical signalling - basically held in position by the interlocking with the protecting signals(s), if such a signal was returned to danger while a train was passing over the points there operating lever would be free to move unless it was also locked by a Clearance Bar. At facing points without an FPL mechanical detection would provide a degree of resistance to the signalbox lever being moved.

 

3. Facing Points in the days of mechanical signalling - the point lever would be held in position by interlocking with the relevant signal lever(s) in addition a further safeguard was the Facing Point Lock Bar, this would be held in its 'down' position by the wheels of a passing movement thus preventing it from lifting and thereby allowing the FPL plunger to withdraw from the port in the stretcher bar - the drive to the Facing Point Lock was taken through it. The Facing Point Lock engaged a plunger with ports (sometimes only for one position) cut into the front stretcher and this physically prevented the switches from moving. Also the detection would provide some degree of resistance to lever movement but was superfluous in this case as the FPL and Lock Bar physically locked the points as a train passed through them.

 

Facing Point Lock Bars were gradually displaced by providing a track circuit which operated an electric lock on the signalbox lever working the FPL, but no physical locking other than the FPL at the points themselves.

 

4. In power signalling the locking is achieved by track circuit controls.

 

One thing worth adding is that the correct working of signals also prevented incorrect or accidental movement of points operated from a signalbox - as far as passenger trains were concerned (not so good on Goods Permissive Lines )

 

Edited to remove my sloppy use of the term 'depressed' (by a passing wheel) of a bar which is already in the low position when the points are locked and which has to be lifted to release the FPL bolt from the port. Thanks to Flying Signalman for detecting my sloppy wording.

[Camberwell Green]

At one time there was a limit on the distance between bogies on longer vehicles to ensure that at least one wheelset was depressing the bar at any one time. At least one of (I think) the LMS dining cars had an odd appearance as a result as the bogies are set too far in aesthetically speaking.

 

The LNWR certainly had some postal vans that suffered from this problem when they combined pairs of older vehicles on a new bogie underframe.

 

As this demonstrates whilst the fouling bars were initially were tailored to the longest vehicles then in operation the fouling bar length then constrained future development of rolling stock.

[The Stationmaster]

The LNWR certainly had some postal vans that suffered from this problem when they combined pairs of older vehicles on a new bogie underframe.

 

As this demonstrates whilst the fouling bars were initially were tailored to the longest vehicles then in operation the fouling bar length then constrained future development of rolling stock.

 

Fouling Bars were not there to lock facing points - the things used to lock facing points in position were Facing Point Lock Bars; Fouling Bars had a different purpose.

[The Stationmaster]

What era of signalling technology are you asking the question in relation to?

 

In "modern" systems the interlocking will not allow the points to be commanded to move if the train detection sections (track circuits or axle counters ) that include the points are occupied. I say "modern" because the track circuit is quite an old technology now. However, before that an alternative method was the fouling bar; this was a device fitted to the inside of the rail in the vicinity of the points which would be depressed by the flanges of the wheels of any vehicles stood over it (like an elongated treadle) and when depressed prevented the points from being moved.

 

Is it now permissible to use axle counters to detect the presence of vehicles in pointwork - have the Principles been eased in that respect?

[Guest Natalie Graham]

I am thinking in particular of situations when a train would often be stopped over a set of points (for example when there is a crossover part way along a platform) and the points being controlled by a signalman who can not see the situation from his box.

 

I don't know if things changed later but certainly in the pre-group era the requirement was that a signalbox should be sited so that all the points it controlled were visible from the box. Hence the provision of two boxes at some comparitively minor stations with passing loops.

[TheSignalEngineer]

I think that there is also some confusion here between facing point lock bars and depression bars.

 

Facing point lock bars (and fouling bars) normally lie below flange level. They have to be mechanically moved through a position that cannot be achieved with a wheel flange present, and if this is not possible the points cannot be unlocked or moved.

 

A depression bar normally stands at rail level and is depressed by the flange to break an electrical circuit which detects the presence of a vehicle. They were often used at the stop block end of platforms to detect the presence of a vehicle on the blocks where a track circuit would not work reliably due to dirty rails.

[flyingsignalman]

Is it now permissible to use axle counters to detect the presence of vehicles in pointwork - have the Principles been eased in that respect?

 

When control of the Wirral lines of Merseyrail were transferred to the IECC (in September/October 1994) there were several axle counter sections that had pointwork in them.

[royaloak]

Is it now permissible to use axle counters to detect the presence of vehicles in pointwork - have the Principles been eased in that respect?

Yes and they work on points as good as they do on plain line.

[The Stationmaster]

When control of the Wirral lines of Merseyrail were transferred to the IECC (in September/October 1994) there were several axle counter sections that had pointwork in them.

 

Understood but presumably, from what I can recall, at that time the points themselves were still required to be protected by a track circuit - I thought that requirement lasted a lot later than 1994 as t was being talked about on schemes I had a little to do with some years after that (unless someone was going in for belt & braces?).

[phil-b259]

Understood but presumably, from what I can recall, at that time the points themselves were still required to be protected by a track circuit - I thought that requirement lasted a lot later than 1994 as t was being talked about on schemes I had a little to do with some years after that (unless someone was going in for belt & braces?).

 

I'm not sure of the exact date but ever since it was re signalled as part of the WCML modernisation, Euston station throat has been totaly reliant on axle counters for train detection (which caused chaos one morning when power was lost for an extended peroid leading to all axle counters to lose any record of where trains were).

 

I think though to some extent it comes down to regional prefrences and I believe that while the ex Southern region doesn't mind using axle counters for plain line, in junction areas traditional track circuits are prefered

[Fay Singpoint]

I'm not sure of the exact date but ever since it was re signalled as part of the WCML modernisation, Euston station throat has been totaly reliant on axle counters for train detection

 

The only axle counters at Euston are in Primrose Hill Tunnels. Everything else is track circuits.

[Southernman46]

Three notable accidents worth reading about where points have moved when they shouldn't have to derail a train / cause a collision are -

 

1973 - Ealing Broadway

1969 - Connington South (near Peterborough)

1927 - Hull Paragon

[62613]

I thought the Hull Paragon accident was caused by a signalman returning signals early behind a train, and leaving a set of points able to be moved by another in the same box, with the result that they were, and two trains ended up on the same piece of line in a head-on collision.

[Southernman46]

Yes, but the the accident report describes the type of equipment (a depressed bar ? there was about a 33 yard "hole" in the interlocking that enable this to be done IIRC ?) that locked the points and prevents the signals being re-cleared etc.... just to give the OP some background info that they were after.