Level Crossings and Train Lengths

[whhistle]

On some level crossings, I guess there's some sort of sensor on the track to detect when the last section of a train has passed, so the barriers can be opened?

Can anyone tell me a little how this works?

 

[dk1]

On some level crossings, I guess there's some sort of sensor on the track to detect when the last section of a train has passed, so the barriers can be opened?

Can anyone tell me a little how this works?

Normally track circuits, treadles or axle counters.

 

[alxndr]

The exact set up depends on the type of crossing and the type of train detection in use at that crossing, but will be using either a combination of track circuits and treadles, or axle counters. These will be used to divide the track into sections so that the location of the train can be determined, with the interlocking allowing the crossing to raise when the sequence of sections going occupied and then clear indicates that the train has passed over the crossing.

 

[driver9000]

If you look at the exit side of some automatic crossings you'll see a small box in the four foot with a leg coming out of it towards the rail. This is a treadle which is used to tell the crossing the train has left and set the opening sequence to allow road traffic across. At the "strike in" point on approach which will vary according to permitted speed a similar treadle will start the crossing sequence before the train arrives. Other crossings will use the occupation of track circuits or passing axle counters.

 

[MarkyT]

If you look at the exit side of some automatic crossings you'll see a small box in the four foot with a leg coming out of it towards the rail. This is a treadle which is used to tell the crossing the train has left and set the opening sequence to allow road traffic across. At the "strike in" point on approach which will vary according to permitted speed a similar treadle will start the crossing sequence before the train arrives. Other crossings will use the occupation of track circuits or passing axle counters.

Treadles can't reliably detect the absence of trains, as if a vehicle was stationary over the crossing the damped actuator arm could rise between wheels if an axle wasn't stopped directly overhead. The exit or 'strike out' treadle is always used in combination with track circuits, the logic being the crossing can open once the approach track circuit goes clear after the strike out treadle has been operated. There are both traditional mechanical treadles and more modern inductive sensor equivalents in use. Mechanical treadles can have a single arm, or two arms for unidirectional operation.
A single arm example: https://commons.wikimedia.org/wiki/File:Rail_treadle.jpg

 

[edwin_m]

This accident report talks about what is done on modern crossings, and what wasn't done on an older one resulting in the accident.

https://assets.publishing.service.g...ile/729309/R082018_180719_Stainforth_Road.pdf

a car collided with the rear-most wagon of a stationary freight train at Stainforth Road Automatic Half-Barrier level crossing, near Doncaster. The crossing’s warning equipment was not operating and its half- barriers were raised when the car approached and entered the crossing.
...
The crossing’s control circuits dated back to its original installation in 1974. The control circuits had not been modified to incorporate later features which prove that trains are clear of a crossing before it re-opens.

 

[Varney]

Ooops!

 

[dk1]

Ooops!

That looks like an ABCL so will time out.

 

[Foggycorner]

Only an idiot would drive onto a crossing with the train still in the way, it is a yellow box so you don't enter unless exit is clear and with a train in the way it's not clear is it

 

[whhistle]

Thanks all

I guess some track circuits have extra time added?
So after the passage of a train, perhaps there's a 4 or 5 second countdown before the barrier opening sequence begins?

I guess also if a train stops with one bogie on one track, and one bogie on a closer piece of track, which is split by a track circuit then it's completing the circuit and the signallers panel (old panel) would show two lights, IE, the train being in both sections?

 

[edwin_m]

Thanks all

I guess some track circuits have extra time added?
So after the passage of a train, perhaps there's a 4 or 5 second countdown before the barrier opening sequence begins?

I guess also if a train stops with one bogie on one track, and one bogie on a closer piece of track, which is split by a track circuit then it's completing the circuit and the signallers panel (old panel) would show two lights, IE, the train being in both sections?

I don't think there are timers involved in starting the crossing opening sequence. The report I linked to above discusses how modern practice would be to put a short track circuit across the crossing, which keeps the barriers down if it is occupied but won't start the crossing sequence if it becomes occupied (as it may be shorted out by something happening on the road). The sequence must be started by one of the track circuits or treadles on the approach. Yes if a train straddles two track circuits both will show occupied.

 

[Varney]

Only an idiot would drive onto a crossing with the train still in the way, it is a yellow box so you don't enter unless exit is clear and with a train in the way it's not clear is it

It can happen though...

https://www.gov.uk/raib-reports/report-08-2018-collision-at-stainforth-road-level-crossing