... and on the West Coast Main Line too. It seems to me that the signal spacings are very stretched out, maybe because they were for 140mph or whatever, and the max is 125. I guess part of the benefit was the cost saving from fewer sections. Consequence of a lot fewer, longer signal sections is that when things go wrong & congestion occurs the tailback is an awful lot longer, blocking junctions further back etc etc.
I wonder if it would be more resilient and have more capacity if they were all halved in length?
Prior to the 1960s the WCML had mostly absolute block signalling controlled by signal boxes which were spaced quite unevenly because they needed to be at stations or other places with points. There were some intermediate signals but overall the length of the blocks was quite irregular and effectively the longest block limited the capacity of the entire route. So the multiple-aspect signalling installed just before electrification would be a great capacity improvement because it evened up the block sections. It also shortened them, because under the old system the block length could not sensibly be shorter than the braking distances of the trains but under four aspect signalling it can be as little as half the braking distance. However, trains at maximum speed would then have to be separated by more than one block system, so the benefit of four-aspect signalling isn't as much as a doubling in capacity. A further benefit is that the equipment works automatically, not requiring time for the signalmen to communicate by bell and block instrument before and after each train passes.
In theory this could be taken further by adding more aspects and spacing the signals more closely, but even in theory there is a diminishing return (five-aspect signals would be spaced at one-third of braking distance, six-aspect at one-quarter etc). There are also practical limitations such as the impossibility of placing signals at junctions, midway along platforms or where they are not easily visible. So the theoretical minimum spacing becomes harder to achieve with more aspects and shorter blocks. In addition a driver is more likely to make mistakes when faced with a large number of possible aspects, particularly when the signals are approached at a higher speed and more closely spaced.
Moving block allows a different concept. Essentially a certain distance is reserved in front of each train, and it is allowed to run at the speed which it is able to stop at within that distance. This avoids the need to "round up" the train spacing to a whole number of signal sections, allowing the minimum spacing between trains at maximum speed to be closer. Perhaps more importantly it also means that trains running more slowly or with better brakes can run safely at a shorter distance behind the train in front. This is a particular benefit on a mixed traffic line because the risk of driver error means trains are normally timetabled to run on green signals even if their stopping distance would allow them to run at their maximum speed on double yellows. As I suggested above, moving block is equivalent to a large number of very short block sections, with a similarly large number of signal aspects.
Having said that, moving block is proven technology for metros where all trains have the same characteristics, but less so for main lines. ERTMS level 3 will allow this to happen but does not yet exist. And it makes very little difference to the capacity disbenefit that comes from some trains stopping at particular stations where others don't.
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It has worked well on the Central line , long before ATO. I remember in the late 60 s when I worked up there you could see the next rain at several stations
Indeed. Underground lines with classic signalling have featured "closing up" signals for many years. The signals are spaced more closely near stations allowing a following train to approach and occupy the platform in the minimum time after the previous one has left. This can even include mid-platform signals. The limiting factor is making sure that if a train passes the signal at danger the trainstop can bring it to a halt before it hits the one in front.
I do not understand what all the fuss about the Thameslink improvements has been all about, the speeds on the central section are modest I would think they could have just put in lots of closely spaced 4 aspect signals and saved loads a money on the automatic signal system they are putting in,
That is exactly what they are doing, and you can see the mid-platform signals at the stations. This is a fixed block system with the shortest blocks that are possible under four-aspect signalling, taking account of the low speed and the braking performance of the trains to be used.
On top of this the new train fleet will use ATO. This doesn't reduce the minimum spacing. However it does produce a capacity benefit because all trains will accelerate and brake in an identical manner (eliminating the differences cause by variations in driving style between drivers) so they will take exactly the same time from station to station and are less likely to catch each other up.
Crossrail central section has a true moving block system similar to those on the Underground.
