Why are there blue aspects on the jubilee line

Blue aspects are similar in function to the lunar white aspects used on the Central Line - they indicate that a given number of signalling section blocks are clear beyond the signal, but that the line is not clear all the way to the next signal.

Oh, and they were going to be white, like on the Central and Victoria lines, but the HMRI 'Railway Safety Principles and Guidance' document (part 2D - signalling) published in 1996 notes that white lights are not suitable for main signal aspects - so blue was chosen instead.

As I understood it, blue lights were put in place due to the phased-in nature of the Jubilee upgrade and signified that it was clear to enter the TBTC zone ahead. A green aspect couldn't be used as that essentially means to continue to the next signal, of which, if TBTC is in operation, there shouldn't be.

Aren't the blue aspects just a temporary thing while there is mixed mode signalling going on - once TBTC is fully operational and the legacy signalling equipment is taken out there will be no need for signals, route indicators or even blue lights as the Train Operator will get any information from the driver's console?

I thought it was the lamps that are coloured with clear lenses.

So does anyone know for certain?

Central is a fixed block ATO/ATP system by Westinghouse.. just far more advanced than that of the Vic. Explained here: www.davros.org/rail/signalling/articles/central.html

Oooh. I dunno. There are more codes, but only using a different technology. Vic. moving overlaps are quite elegant.

No, old school. ;D

You can swing and extend overlaps with conventional TCs, either jointed or jointless. Moving overlaps are just matching the safety bubble to the front of the train. Granted, it is rather coarse compared to more modern microblocks, but the overlap can still propagate forwards.

If I can successfully paraphrase my notes, I'll add a bit more later.

<this is vaguely on-topic as the principle of moving overlaps is translatable to the Jubilly installation>

Essentially, the principle of moving overlaps is to maintain full speed running for as long as possible - in some locations a full speed approach would result in the second train being tripped too soon behind the first. It is an engaging peculiarity of the Victoria system that fixed signals/headway posts are sited part way along a full speed overlap, then there are the 'braking from full speed' command spots. The moving overlaps - determined by the relationship between the departing and arriving train - are independent of the usual braking command spots. What makes these quite so elegant is that the geographical location and relationships of the track circuits themselves determine whether or not to move the overlap. It is a self-comparing circuit and able to control the approach to a station just through the four command codes in the track circuits without using the spots.

The command spots are used to 'smooth out' the braking rate of the train, so there isn't a jerky transition between the codes - sometimes when the conditions are right there's no need for a command spot, the track circuits and code generators will do their thing automagically. ;D


It all hinges on what you mean by 'traditional' TC design: single rail/double rail/AC/DC/coded/uncoded/transient/polarised/frequency-shifted all of which could be considered as traditional. I personally like the theory of transient track circuits (they're a bit like an approach-lit signal) and were used on the M&GC territory. However, another story for another place, perhaps.

Right - so it is not an aspect - it *is* an indicator!

No. Its an aspect, the signal is capable of displaying a red aspect.

Not sure what guidance notes you're referring to, but on the Jubilee (and later on the Northern) blue aspects are very much one of the aspects available on the *signals*.

May I have an un-educated stab at this?

The blue aspect is neccessary as I see it for this reason. If a train stopped at the red aspect was continuing on a conventionally signalled route, he would get a green. As stated above you cannot show a green to the TBTC train as it denotes clear all the way to the next signal, nor can you black the red aspect and inform the driver to simply follow his TOD because then he is staring at a black aspect and a black aspect should be treated as a red aspect, correct? The only option is to show the TBTC train clear to follow TOD aspect. Merely a summisal but it makes sense to me. What say you fellow signal people?

This does seem vaguely similar to the blue aspect used in Russian metros, but that is a slightly different situation. In that system, it's possible for a train without working cab signals to approach a blue aspect, and it would have to be treated as a stop signal. With TBTC, I presume that the signal system will be able to tell if a train has functional TBTC equipment or not, and only show the blue light to those trains that are actually communicating. This is how it works in NYC and on the SF Muni (which also uses Seltrac), with a flashing green and a white "A" respectively. In that case the blue light wouldn't really be a signal aspect, since the only trains that would see it should ignore it, since they're getting track occupancy information from the TBTC.