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Coupling & Uncoupling


Update added 2 January 2005

The Coupling and Uncoupling of Units and Trains

An operation that every Train Operator needs to know how to do is the coupling and uncoupling of trains or of units.  Although this is only rarely used, it is something that is taught in a Train Operator's stock training and is refreshed each year as part of their stock refresher day.

But when is the process ever likely to be done?  London Underground has not for many years routinely shortened its trains during the service day!

Well, there are two circumstances when this can occur 'on the road' and although neither occur routinely they do happen!  One is should a train need to assist another failed train that can only be moved by means of a 'push out'.  The other is, as does occur, if we have a 'one under' or a Person Under a Train as discussed elsewhere on this site, and the Emergency Services need the train 'split' to allow them easier access to a victim. This is an emotive matter and so whether it would be the Train Operator who has actually already undergone the probable trauma of the incident would depend entirely on the circumstances and - more importantly - if he or she was in any state mentally to do so. It might well be that another Train Operator would be sent to the site to do it.

But what we will look at here are a few images of one train that is being split into two separate parts.  In these photographs it's a C69 Stock train that is being 'split' but the principles are similar for every stock.  A complete C69 Stock train as seen in passenger service comprises six cars, but is made up of three two car units which are coupled together. Each two car unit has a motor car (either a Driving Motor Car (DMC) or an Non-Driving Motor Car (UNDM)) and a Trailer car (T), so it can actually be split into three separate parts.  What is illustrated here is the separation of one two car unit from the remainder of the train.

Before the Train Operator actually physically splits the two units there are a number of steps which he will have taken and these serve a number of purposes such as the electrical and pneumatic isolation of one unit from another and to ensure the security of the two separate parts of the train - remember it may well be that he will be acting without assistance and will, in effect, become responsible for the security of two trains!

Again, the preparation for the event varies from stock to stock and, in the event of a 'push out' there are variations according to whether it is the same type of train that will be acting as the assisting train.  On the District Line we of course operate two stocks and it is quite likely that a different stock train will be doing the 'push out' - it could be that a D78 Stock will be pushing a C69 Stock or vice versa and even possible that an A Stock would either need to be assisted or act as the assisting train.

Unless otherwise credited all the photos and clips used have been supplied by Solidbond.

Before looking at images of the actual uncoupling and coupling process, it's worth just viewing a few photos which I hope will assist you in understanding some of the terminology that I'll be using as this section progresses.

This is a view of the front of the DMC of a C69 Stock train. Working from the left, the first thing to note is the yellow coloured handle. This is the Tripcock Isolating Cock (TCIC).  As shown it is in the normal (Open) position - if it were to be moved to the Closed position the tripcock would be cut out and inoperative. Moving right are two items just below the step from the cab door. The upper is the buffer spring, the lower is the Wedgelock coupler which has other items of equipment fitted to it. To the right of the picture is another cock, this time the Dead Man Valve Isolating Cock (DMVIC). The positions are similar to those for the TCIC, but this does not come into effect in this procedure.

Closer detail of the Wedgelock coupler, with the buffer spring above. At the bottom left and right of the device are the stabilisers which allow the couplers to align correctly. Immediately above these are the 'Dutch Ovens' that contain the electrical connections - these open automatically when the units draw together and allow electrical continuity between the units. To the centre are the 'tongue' and the 'throat' which are the mechanical connections. These are held in place by wedges which are air driven to hold the units together.  Finally below these are three valves which provide the pneumatic connection of the two units. So, thus we have mechanical, electrical and pneumatic coupling of the two units.

As mentioned above there will two motor cars involved in this process - one will be a 'conventional' DMC and can be driven from the normal cab, but the other is an UNDM which has no cab in the conventional sense.  So, the train will be propelled from the shunting cabinet as shown above.  Essentially this is an 'abbreviated' cab which has all the controls necessary to move a unit over a relatively short distance.

This is the coupler switch located in the cabinet in the drivers cab.  It has three positions, but to allow the units to be uncoupled would be rotated to this position.

Again a coupler switch, but this time located in the UNDM. Its purpose is identical.

All these above images have been reproduced from London Underground training material.

So, now we look at photos of the two 'units' as the process unfolds. The units have already been uncoupled and are about to be coupled back together.

Looking towards the UNDM you can see that the intercar barriers have been detached and that the UNDM has it's own whistle, windscreen wiper and lights. The bracket just to the right of the door is used to mount a handlamp, should this be required for any reason.

The DMC to which the UNDM will couple. The TCIC can just be seen, and you will note that it's in the 'Cut Out' position.  If this were not the case when recoupled the train would be 'middle tripped' when the Tripcock passed the first trainstop, as this would have raised by the time the middle of the train reached such a device.  One of the isolations an assisting train must do is to cut out its front trip, as this is then of course the middle trip of the coupled trains.  And, similarly, when the procedure is completed and the trains uncoupled once again the trip must be cut back in.

Before proceeding further, the Train Operator must ensure that the coupler itself is aligned to correctly meet with its counterpart on the other unit.  These couplers are heavy and inevitably dirty, and need a fair bit of a push to get them aligned! You may just be able to make out that the Dutch Oven to the right hasn't fully closed, and the electrical studs behind are just visible.

Couplers now aligned, the UNDM is 'opened up' as indicated by the white light showing, and is ready to be moved forward.

On reaching this point the Train Operator will check that all is correctly aligned before finally shunting the two units together.

Finally our train is back together.  Before moving the newly recoupled train any further, the Train operator must carry out a brake test to ensure that he has a full complement of brakes at his command.

The following couple of video clips give a good idea of what the process looks like from within the UNDM as it is parted from and then rejoined to the rest of the train.

The process of uncoupling the UNDM form the rest of the train.

The process of recoupling the UNDM to the rest of the train. The 'humming' sound that can be heard is the compressor running, and you will note that the UNDM has to be propelled quite firmly into the rest of the train to recouple it.  This is to overcome the spring action of the buffer spring.



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