Rear tripcock use

[jimbo]

The March Underground News has an article on the history of tripcocks on trains. I have never really understood the importance of the tripcock at the rear of trains, which risks a train becoming back-tripped and brought to a halt. The tripcock hangs from the front right corner of a train, and will halt a train that attempts to pass a signal at danger by striking the train-stop linked to that signal. Since trains can work in either direction, there is a similar tripcock at the diagonally opposite position on the other end of the train.

It seemed to me that the tripcock at the rear of a train was just a nuisance, and should not be able to halt a train. This seems to have been the policy adopted by the designers of 1972TS, who adapted the design of the first Victoria line trains to run under conventional signalling. However, when procedures for dealing with failures were documented, a major weakness was revealed. If a driver was unable to drive from the leading cab, they went to the rear cab to drive in reverse. A lookout was appointed to ride in the leading cab and report by train phone to the driver at the rear. This lookout might be the train guard, or a co-opted member of station staff, but in any case someone less familiar with the layout of the line and signals than a regular driver. At such a time the driver operating from the rear cab had an operating but pointless tripcock, whilst the leading cab was not the driving position and therefore had no tripcock protection! I believe this was subsequently corrected, and no further train orders followed this principle.

However, many years back in the lead up to the evening peak we found that trains were being back-tripped entering Marble Arch westbound platform. To minimise delays it was decided to advise train guards on 1962TS to cut-out the rear trip at Bond Street, and then reset the trip and cut it back in at Lancaster Gate. I queried why we didn't leave resetting the trip and cutting it back in until the train reached its destination. Resetting and cutting in over a tube station pitted road is not easy and added to the delay of each train. I thought it better to use reversal time at the end of the trip, but I was told that we were not allowed to run trains with the rear trip cut out. Was that true back then, and is it still true?

[Dstock7080]

I thought that rear trips could be cut-out in that situation so long as they were restored promptly after reaching a terminus?

A rear trip activation on D Stock wouldn’t stop the train and only a reminder light was illuminated in the front cab.
S Stock however have reverted to having the rear trip active at all times, very retrograde!

[revupminster]

Of course the rear trip is the front trip when the train reverses. Trains get rear tripped generally at a reversing point if the train stop does not lower automatically or air leaks out and the train stop rises or by an obstruction on the line such as a key from a chair. Single line working also required the rear trip to be cut out.

[t697]

There are or were occasional locations where a wrong direction signal and trainstop are provided. E.g. there's one at Rayners Lane to allow a shunt from the SB platform into the siding. Pre ATC there was one at the south end of Baker St platform 2 for a signalled move on to the Circle line. Platform 2 is slightly uphill in the normal running direction. Therefore the wrong direction trainstop gave rollback protection when the signal was at danger. One reason why the rear tripcock can apply the brake.
The D78 stock was modified as described by Dstock7080 to address a practical operating difficulty we won't go into on here and hence is an exception to the rule.

[philthetube]

There are reasons for having the rear trip operational, however, the 95 stock did not have a rear trip reset button in the front cab and on the S stock the same button resets front and back, resulting in a time delay after resetting before full speed can be achieved. To me it would be sense to be ble to reset the rear trip independently without inhibiting forward movement.

[North End]

Mar 23, 2021 2:01:13 GMT philthetube said:

There are reasons for having the rear trip operational, however, the 95 stock did not have a rear trip reset button in the front cab and on the S stock the same button resets front and back, resulting in a time delay after resetting before full speed can be achieved. To me it would be sense to be ble to reset the rear trip independently without inhibiting forward movement.

The tripcock setup on the 95 stock was odd. The tripcock reset was in the form of a rope, which drivers on occasions had difficulty resetting. Meanwhile the tripcock at the rear end tended to reset itself if activated - so you’d get the brake application, but the train could be driven immediately after stopping with no further intervention required, or at least this happened most of the time! No one seemed to quite know whether this behaviour was intentional or not.

I was once told by someone in Alstom that the system behaved according to how much of a hit the tripcock took (ie the faster the train was going the further the arm would travel, which would influence whether it would latch or not). I’ve no idea what the actual design intention was. This wasn’t an issue at the leading end, though there was an issue at one point earlier in the stock’s life.

Allegedly the whole tripcock mechanism on the 95 stock was an afterthought, though again that’s dubious as there was surely always the expectation that resignalling would be some time after fleet replacement. One way or other, I think everyone was pleased to get rid of the tripcocks once TBTC came in.

[philthetube]

While not an afterthought, it was a temporary fix and was not expected to bu used for more than a couple of years. There was at least one occurrence when a trip re set when the train was moving forward, this caused big issues and a redesign.

I may be wrong but I feel convinced that there was a trip reset button in the cab. I haven't been in one for 20+years.

[North End]

Mar 23, 2021 5:41:35 GMT philthetube said:

While not an afterthought, it was a temporary fix and was not expected to bu used for more than a couple of years. There was at least one occurrence when a trip re set when the train was moving forward, this caused big issues and a redesign.

I may be wrong but I feel convinced that there was a trip reset button in the cab. I haven't been in one for 20+years.

Not to my knowledge, just the awful rope arrangement. It certainly wasn’t unknown for the tripcock to not reset, or even for the rope to snap! Looking back it’s incredible the arrangement ever existed.

No idea what was provided on the 96 stock, could they have had the button?

[t697]

95TS and 96TS both had the same reset rope arrangement. 96TS was supposed to have gone ATC very soon after build with the original intent JLE ATC signalling that got cancelled. Quite a history of that isn't there!

[philthetube]

I accept that my memory plays tricks then