trip cocks and train stops

The only tripcock arms on the trains are on the Driving Motor cars - cars that can appear at the front. So for any stock which doesn't have cabs in the middle of the train, obviously there's only the one at the front and the one at the back. For trains with cabs in the middle, the tripcock arms on those cars are not lowered when they're in the middle of the train. Obviously they must be lowered if the car is put on the front of a formation.

Now, there is only one tripcock on a car, it's on the right (in the direction of travel). The train stop is on the right, in the direction of travel. So the rear tripcock arm of a train can't hit the trainstop because it'll be on the left. When the train reverses and the back car becomes the front, its tripcock arm is now on the right in the direction of travel, if you see what I mean. Because the train is now the other way around. The tripcock arm is always on the right hand side of the car, but obviously the front car and the rear car are opposite ways around.

What this does mean, is that if you have a wrong road signal, its trainstop will be on the right for moves using that signal. Which is on the left for trains travelling in the normal direction. Which means that their rear tripcock arms would hit the trainstop of the wrong road signal as they travel along in the normal direction. And so the trainstop lowers automatically as a train approaches in the right direction, to avoid the train being "rear-tripped".

Occasionally trains are rear-tripped by debris or other causes. They can also be tripped at the front by mistake as well.

Edited to add a crude diagram:


Obviously the red rectangles projecting out of the trains are supposed to be tripcock arms, although they don't reach out to the side, they hang down below. The one near the signal represents a raised trainstop, which would be hit by the tripcock arm of the front car, but cannot be hit by the one on the rear car. The yellow bit represents the front of the car. You can see that obviously the front car and the rear car are different ways around so their tripcock arms, which are always on the right from the point of view of someone looking out of the front window, are on different sides of the train. Obviously, when the train shown reaches its destination, reverses, and the back becomes the front, the tripcock arm which is currently at the back and on the left of the train, from the point of view of someone looking out of the front window, will become the front arm and will be on the right.

I believe they are isolated and then left in the tripped position. When a train is reformed or being prepped, the person prepping then goes around and checks the tripcocks are tripping the train as required.

Someone please correct me if I am wrong.

The Rear Tripcock operation event once happened on a Northern line train around the turn of the Century, where the driver fell asleep at the controls, and the train slid back a short way, luckily the then new 1995 trains had rear protection, so the rear tripcock applied the emergency brake in the usual way, once the train slid back, and came into contact with a raised train-stop-arm in the rear signalled section.

uncoupling the auto couplers also resets the tripcock(s) to the "armed" position

In fact it wasn't the rear tripcock that was operated but the front one, due to the relative positions of block joint and trainstop at a specific signal.
Subsequently a system of Runback Protection was fitted to all the passenger fleets still running on tripcock/trainstop lines to address the risk. All the lines with ATC have runback protection inherent in the ATP schemes.