This page will be used to look at matters which are generic to London Underground Operations, and by the size of the topic do not justify a page in their own right.

Where is Current drawn from when trains are in depots?

This question has arisen as several readers have noticed that on the inside roads in depots there are no traction current rails - only running rails, so the normal current shoes are 'off juice'. So where does the train draw its power from, should it need to be moved?

Firstly, it is worth considering how the 630v DC current is distributed around the train. The following diagram is for the D78 Stock train, but most London Underground stocks are configured similarly, though of course the number of cars varies from stock to stock, as does the physical location of some equipment.

(clicking on this image will produce a full size version)

It is worth looking at how the train is made up. Each six car train is actually two units, each of three cars coupled together. Our illustration shows two 'single ended' (a driving cab at one end) units coupled together; there are a number of 'double ended' (a driving cab at both ends) on the fleet, but for the purpose of this section I'm using the single ended configuration.

You will note therefore that when the two units are coupled together all the equipment is duplicated and you may also notice that between the two units there is no linking of the 630v DC current distribution as shown by the red and blue lines which represent positive and negative respectively.

For the purpose of this discussion, what we're interested in are the 'Shed Receptacle Boxes' which can be seen on cars 1,3,4 and 6. These are effectively like household sockets, though they work in reverse in that instead of distributing current, they receive it, in place of the normal shoes.

Externally this is what the Receptacle Box looks like (this was actually from the Museum's 1938 Tube Stock unit, but the equipment is generic):

This is the box as you may well see it when out and about on the Underground. This is in fact a hinged flap. When raised it looks like this:

(Thanks to my colleague Wayne Price for modelling his fingers in this picture!)

So, that's where the train gets its power, but where is it supplied from? Current is supplied from overhead cables (called Shed Leads) which are suspended from tracks above the trains as illustrated below.

Again, this is a 1938 Tube stock unit, but you can see the cables inserted into the train, if you follow the line of the leads towards the roof you can see the tracks on which they are able to run; another set of tracks is visible to the left, above the D78 Stock.

In the following photo by Chris Cobley taken in Neasden Depot the leads can be seen in the foreground hanging on their storage hooks.

Nowadays only depot staff insert and remove the leads and move trains with the leads attached. When a Train Operator receives a train for service it will have been drawn forward so that the front DMC is 'on juice' and therefore is receiving current conventionally and the leads to the rear of the train should have been removed. However, the Train Operator preparing the train for service must ensure that the leads have been removed; if a train leaves the shed with the leads attached substantial damage will occur to both the train and the shed structure.

To help avoid the situation on the D78 Stock the Train Monitoring System (TMS) senses the presence of a shed lead and warns the Train Operator by means of an illuminated light and a 'sonalert' (a audible warning).

If there was a shed plug inserted in the train the 'Shed Plug In' light would illuminate

Update added 3 January 2005

Dealing with Seized Motors!

Obviously not an everyday occurrence, but something that does happen is where the traction motors on a bogey become seized. This can happen through a variety of circumstances, and needs to be addressed causing the least amount of damage to the train.

The indications to the driver is that the train will be 'sluggish' as not only has he lost the motors on all or part of a motor car, but the wheels themselves will be locked and he will be dragging them! This obviously causes 'flats' to the wheels which are locked, though at this stage, this is the least of the problem!

Of course, seized motors is only one possibility - brakes failing to release would be another cause for the same symptoms. However, this is more easily overcome - there are means whereby locked brakes can be released, though this will mean of course that the train is now short of braking capacity.

But seized motors are more difficult to overcome, and can only be fully addressed in the depot. So how can the train be got to depot? Initially the train will be driven to the nearest depot if possible, but more likely the Controller will want it out of the way as soon this can be done, so the chances are it will be taken to a siding or even a bay platform, though this is less preferable as it will mean that this platform is effectively going to be out of use for quite some time!

The Emergency Response Unit (ERU), of whom mention is made in a number of places around this site, will attend the train. The affected part of the train will be raised by being jacked up and the affected wheels placed on 'skates'. These are very similar to the roller skates after which they take their name, and allow the locked wheels to be lowered into them and then to move without causing further damage to the wheels and other parts of the gearing and axles, though these will be replaced anyway - even if only as a precaution.

The following photos were taken by the ever-resourceful 'Solidbond' on his mobile phone recently when there was a C69 Stock train in Parsons Green sidings with seized motors which had to be moved back to Hammersmith depot for repair. Obviously this is quite a distance, and this system had to be employed to achieve the move.

I've tried to keep the photos as close as possible to their original size for the sake of clarity, and I think that they're pretty much self-explanatory!