Tripcock requirements

Trains are required to pass over a tripcock tester normally once a trip. However, all a tripcock tester does is to check that the trip arm is physically present and in gauge. Personally I think it's incoceivable that a trip arm could be damaged such that it won't operate a tester and the train not be tripped.

For entry into tripcock areas, they are valuable for double checking that the tripcock system is cut in and lever reset. However, LU passenger trains trains are hard to move at any speed with the tripcock cut out.

I'm not quite sure what you mean with your last sentence there. Are you saying that you can't conceive of a situation where all of the following are true
* the tripcock arm is damaged
* the tripcock tester does not detect anything wrong
* the triocock system fails to trip a train when it passes a raised trainstop?

i.e. if the tripcock is damaged it will either fail the tripcock test or still work as intended (damaged but not broken)?

Operational staff, so even a controller or signalman travelling home. (same with a defective deadmans)

Basically the tripcock tester checks that the tripcock arm is in gauge in two dimensions such that it will be struck by a raised trainstop head. Even an 'in gauge' tripcock arm can be close enough to the tolerances for a train to be tripped in some circumstances.

There are several variables to be considered when keeping tripcock arms in gauge. Rail wear affects the trip 'height' dimension as does re-railing so trainstops are gauged regularly and height adjusted to suit rail wear as required, when new track is laid the adjacent trainstops are reinstalled and 'put to gauge'.
Sleepers and fixings also wear and it is possible for trainstops to move away from or closer to the running rail with wear and vibration but again such movement is picked up on regular maintenance checks and corrected as a matter of priority.

Of course tripcock testers themselves are also liable to go out of gauge for the same reason as trainstops due to rail wear or replacement but the tolerances are smaller and so it is more likely for an 'in gauge' tripcock to fail the tripcock test if the tripcock tester has moved relative to its gauge lines.

At the end of the day and relative to the running rail height not all trainstop heads will be at the same height above the crown of the running rail but they should all be within the allowed tolerance i.e. within gauge. It would be very unlikely for a trainstop head to move so far away from the running rail as to be unable to operate a tripcock in normal operation and with fair wear and tear, neverthelss the trainstop head is made wide enough to take account of the possibility.

Of course it is possible for trainstop heads to be decapitated by defective trains, loose pickup shoes being the usual culprit, but this automatically maintains signals in rear at danger by mechanical and electrical means inherent in the failsafe safety signalling devices and circuitry.

What is the status of the tripcock at the back of the train (for when it goes the other way): is it 'switched off', as it were, so that 'wrong direction' signals are ignored, or are things arranged so that their trip arms are lowered?
If the former, then damage without tripping is possible.
Is any sort of visual check of the trip cock part of the driver's routine on taking over a cab?

To make it a bit clearer, trip cocks at both ends of a train are always cut in [and are therefore always operative). Where cabs are in the middle, the trip cocks will be cut out. As 21146 says, relevant train stops lower as required ensuring trains are not back tripped.

Back tripping is however possible for a number of reasons and when it happens different stocks act in different ways - some will cause no bother until a driver changes ends at a terminus (the back thus now becoming the front), others will knock out the train line air giving a burst (essentially just like being front tripped). In the case of the latter, it has to be dealt with there and then.

As for depot checks when preparing a train for service, I don't believe there is a requirement to check the trip cock arm is there (if there is I've not been prepping properly for the last 5 years) - if it was missing surely you ought to have a burst!! - but it is most certainly part of the train prep to ensure the trip cock is cut in.

It is part of the Fleet Maintenance daily Prep to actually operate the tripcock and confirm it applies the brake, so the presence of the lever is of course then checked. The Train Operator's Prep is as you said.

Not much to add really - the positioning is different but that's about all that the Train Op should see that differs from normal.

At Stonebridge Park, however, tripcocks are tested as trains leave the depot. However the tester is a Network Rail one and where it alarms to if a train fails the test, I'm not sure...

I don't know how aware or not drivers are made of slow to operate trainstops but there are a few of those about where trains enter service such as WR9 at Northfields and WY3 at Heathrow for instance where accelerating sharply away as the signal clears is liable to trip the train. Basically the lowering time of the trainstop is increased by inserting a length of 1.5" air main GI pipe 'in series' with the heston hose between the GE valve and the trainstop, the longer the length of air main the greater the delay as the pipe has a greater cross sectional area and serves to reduce the instantaneous air pressure which then takes longer to build up and overcome the trainstop return spring.

Back tripping sometimes happens at wrong road trainstops, my thinking is that perhaps this is due to 'swing' as they are normally associated with crossovers and as such a curve is involved and the track gauge although within tolerance is possibly likely to be a fraction wider. Also these trainstops mounted at blockjoints are more liable to move under vibration especially if the trackbed isn't solid. I have seen such trainstops jumping up and down when investigating back trip incidents. Of course one train may be backtripped and 10 go by without let or hindrance and to my mind that is all about the tight tolerances involved, safety first and always!

Well....

When the re-signalling is complete we'll be running under ATP with ATO, so the train stops will be history anyway!!

Drivers are very aware of slow train stops Places like Whitechapel are very much part of the road training.

Take Parsons Green on the District for example - train stops all over the place - back tripping is very easy there as when a release is taken on any signal in the area, they all go back. I don't imagine "swing" has anything to do with it at all as such train stops would normally lower as the train passes over them - it's only in the release situation that it may raise under a train and be an issue.

First I've heard of such an issue!!

What has been an issue of late is pipe connections being loosened by persons unknown such that the trip arm doesn't always go fully down. All you then need is a train with an arm slightly lower than the rest to catch it and Bob's yer uncle. I had that a few months ago - got tripped but I swore the signal was Green....turned out the air pipe had been loosened off and my train had a lower-than-the-rest arm which caught it.