Not in Service

Another nice winter's morning today. I saw the RAT trundling through WOO as I waited for my train into LIS and then, at SOW (South Woodford), I noticed that a train which was not in service was due shortly on the eastbound. As a result we were held short of LES (Leytonstone) at my old friend signal LES 5760. We had quite a bit of a wait because they were reversing this out of service train off of westbound platform 2. I saw it pass over the crossover ahead of us and head on its way to SNA (Snaresbrook):

A diagram of the track layout around Leytonstone showing the signal LES 5760 at which we were held, my train (in blue) and the out of service train (in brown) reversing off of WB2 (westbound platorfm 2) over the crossover, on its way to Snaresbrook.

I suppose they were taking it back to the depot at HAI (Hainault) via WOO. The reason for this is probably that, as you can see, the eastbound track towards WAN (Wanstead) can't be easily reached from WB1 or WB2 (you have to continue on the westbound and reverse it into EB3, holding up the westbound and eastbound. So, any failure that happens between NEP (Newbury Park) and LES on the westbound usually requires you to get the train back to the depot via WOO.

I don't, of course, know what the matter was, but it was being driven back manually. As a (hopefully) interesting aside, the Central line is generally operated by ATO (Automatic Train Operation). The system is not fantastically advanced, but there's a little bit to it, so I won't go into too much detail. Suffice it to say that the line is divided into sections called "blocks". (This is actually true on conventional, manually operated lines, where each block is controlled by a signal). On the Central line (as with most railways, particularly passenger railways) no more than one train can occupy a block at any one time. This is called "non-permissive block signalling" and the idea is that trains are kept a safe distance apart by this method. If a train is already in the block ahead, the following train will not be able to enter that block. On manually operated railways there will be a red (or "danger") signal, which will not clear until the train ahead has left the block (usually there is actually an overlap for safety reasons - the train needs to be a little way into the next block, to allow the following train to stop short of it if the first train breaks down or something like that). The reason for all of this is that the signals act like the driver's eyes. It's often said that signals are like the traffic lights of the railways. In some cases, such as at junctions, this is quite true. In reality, though, the signals are also there to keep trains apart. The point is that a train cannot swerve, nor can it stop very quickly at all. Therefore, one cannot rely on the drivers of trains to keep them apart, because, at full line speed, if you can see a train that has stopped ahead of you, it will probably be too late, you're going to hit it. So, instead, the line is divided into blocks and signals keep the trains apart.

On the Central line, the principle is the same, but rather than using colour light signals, there is a piece of equipment at the start and end of each block which transmits codes to the train. Most blocks are marked by a "block marker board" (or "block section marker") which looks like this:

A block marker board. The TCR refers to the location of the block (TCR = Tottenham Court Road). A refers to the fact that it is an automatic signal controlled solely by the movement of trains and 4155 is its number. (Adapted from a diagram here: http://www.davros.org/rail/signalling/articles/central.html - a great source for more information about Central line ATO)

Some real block marker boards are clearly visible here at the sidings at WOO:

Woodford bay platform (westbound platform 1) with block marker boards clearly visible. (Image courtesy "Sunil060902" (via Wikipedia))

These codes are interpreted by the on-board computer, which responds as necessary. If there is a train ahead, the target speed will be stepped down gradually, so that the train stops comfortably short. Obviously this works in reverse. The target speed is stepped up gradually as the train ahead moves away, so that trains are always kept safely apart and always travelling as quickly as they can, within the limits of the technology at the time. The target speed obviously also changes to reflect any permanent speed restrictions on a given section of track. Thus the train can accelerate and decelerate as necessary, keeping a safe distance away from the train in front, keeping to the line speed limit and the ATO also takes care of stopping in the stations.

The blocks are quite close together, generally much closer than on manually driven railways, depending on how close you need trains to travel together. The closer trains are to each other, the more you can fit on the line, but the slower they can travel (because there's not as much room). They will also be placed around junctions, crossovers, sidings and the like and at platforms to control all of these. In general, at these critical locations (sc. junctions, crossovers, sidings, platforms, etc.), colour light signals are provided as well. These are for trains which are being driven manually. They are generally three-aspect signals (although there are some with only two aspects) and may display red:

A Central line three-aspect signal displaying a red aspect

A photograph of an actual Central line three-aspect signal (located at Roding Valley) displaying a red aspect. (Taken from an image courtesy Mike Knell (via Wikipedia))

At which trains are required to stop. The code for the next block will be stop.

White:

A Central line three-aspect signal displaying a white aspect

A photograph of an actual Central line three-aspect signal (located at Leyton) displaying a white aspect. This image is reproduced here by kind permission of the copyright holder - "Flickr" user "bowroaduk". © All rights reserved by bowroaduk. "bowroaduk" is not affiliated in any way with this blog and has played no part in the production of this post. Their kind permission to use this image does not imply any endorsement of the content of this blog. This image has been cropped from the original image available here. Any requests to re-use this image must be addressed to "bowroaduk"

Which indicates that the next block is clear, the target speed is anything other than stop and the speed limit in the next block is anything other than stop. Thus trains which are being driven automatically can proceed, as can trains being driven in "coded manual" (or "coded", more on this later). Trains being driven in restricted manual (or RM, more on this later, too) cannot.


And green:

A Central line three-aspect signal displaying a green aspect

A photograph of an actual Central line three-aspect signal (located at Perivale) displaying a green aspect. This image has been cropped from an excellent image courtesy of "Flickr" user "R/DV/RS" and the original may be viewed here. The license applying to the use of this image may be viewed here.

 Which indicates that the track up to the next colour light signal is clear and all trains may proceed.

Although the signals are present - and some of them are semi-automatic and are controlled by a signalman - codes are still transmitted to the train just like normal, although the equipment works slightly differently at points and crossings. As we have seen elsewhere in this blog, the Central line also has special junction signals and shunt signals as well.

So, when the train is in Auto, the on-board computer responds to the codes received from the track-side equipment. The T/Op opens the doors and uses the CCTV in the cab to close them safely. The T/Op also operates the two start buttons, which gets the train on its way, when the starter signal is cleared. All other normal operation is taken care of by the on-board computer.

This isn't a rant about NoPO (OPO = One Person Operation) but I just wanna emphasise that there's a lot more to it than that implies. Apart from the safety role one is expected to fulfil by making sure that everyone gets on and off the train safely and the customer service role, the T/Op is also expected to fix any problems which might occur which are in their scope to fix and to be the competent authority in charge of the train and communicating with the control room.

Sometimes, though, it is necessary to drive the train "on the handle" in coded manual mode. In coded manual, the codes are still transmitted to the train, but - rather than being acted on by the computer - they are displayed on a screen for the T/Op to act on. The current speed is displayed as a green bar and the target speed as an orange bar beneath it. If the target speed changes, it will change on the screen. If it goes up, there will also be three chimes going up in pitch, if it goes down, there will be three chimes falling in pitch. If the T/Op doesn't take action in time an alarm will sound and the brakes will apply until the speed has dropped below the target speed, when control will be returned to the T/Op (this is ATP (Automatic Train Protection)).

You might wonder why this is necessary. Well, firstly, some failures can necessitate it. I remember my favourite blogger "aslefshrugged" once told the story of how a fault on his train had meant that it wouldn't travel faster than 40 kph (line speed reaches 65 kph) in auto, but he could achieve full line speed in coded and so drove in coded (full story: here). Also, auto doesn't like rain, as a brief perusal of "aslefshrugged"'s blog will inform you, and it tends to stop wherever it fancies once things get a bit wet, meaning that T/Ops are often forced to give up and drive the things themselves. Another reason is after heavy winds, if it's feared there might be an obstruction on the track, the first trains in the morning will often run in coded and the T/Ops to keep their eyes open for anything, which they can then stop short of and inform Wood Lane about. Also, if there's a disturbance, such as a fight, going on on a platform, T/Ops might be asked to pull in slowly in coded, so that they have a chance of stopping short of anyone who might end up on the track. T/Ops are also supposed to drive in coded east of Leytonstone and west of White City, I believe, on Sundays to keep in practice (most of this is in the open) and, at certain times of day, I do believe they're allowed to drive in coded in these sections if they want, but in the busy, dark tunnel sections, I think you're supposed to go in Auto unless you  have a reason not to. Since the train is still being driven on the basis of codes and ATP is operational, these trains can pass white signals.

Restricted manual is used where there's been a full ATO/ATP failure or if there's a substantial problem on a train (or in the depots, where there are no codes). In restricted manual the T/Op drives by sight in full manual mode. The train is limited to 18 kph and the motors cut out at 16 kph. All signals must be obeyed. Since there are no codes, white signals cannot be passed. This is because they indicate that the codes are clear, the next block is clear, but not the track to the next colour light signal. So, if colour light signals are all you have to go on, you can't guarantee that you won't come round a corner and see a train stopped ahead of you. Even at 16 kph, that's not gonna be a happy ending.

Anyway, a pretty whistle stop tour, but that's manual operation on the Central line in miniature. After that wait outside LES, it was pretty plain sailing and there wasn't much to report on the way back, either. The Hammersmith & City line did have delays on it between LIS and Barking by the time I got back to WOO, but I think I managed to avoid getting caught up in them.