Thanks, that confirms what I thought.
Regularly happens in the Machynlleth - Dovey Jcn section, when the portions to/from Aberystwyth and Pwllheli follow one another. Elsewhere, it would depend on whether intermediate block markers are provided to divide the section.
Hereford is all track circuited within the area the box controls.
Doesn't mean that it's not under Absolute Block regs (which I think it is, albeit somewhat muddied by the fact that it works TCB in one direction but not the other, to the north).
Greenford isn't worked under AB regs in any direction, though. On the other hand, the vast majority of boxes that *do* work AB have at least some track circuits – I can't think of any that don't at least have a berth track circuit in each direction.
No, it doesn't. Once a train has passed the home signal just to the west of Wolvercote North Junction, as it is now named, no other train can enter the section until the first train has entered the double track section at Charlbury.
It shouldn't be – it *really* shouldn't be. The presence of semaphores does not equate to Absolute Block. You can have AB without semaphores, and you can have semaphores without AB. It's an important distinction, and alarming that this misconception is so widespread.
Greenford is one such location! Swinderby, too.
My understanding is that if you want to replace the semaphores with colour lights, you need 3 aspect signals except at the signal giving access to the next section which can be r/g colour light as the next signal will be a distant.
Yes, absolutely. I think we all agree on that. The misconception is that that means that it's Absolute Block, and it's a common misconception, perhaps because of the relative rarity of semaphores on pure TCB (rather more common at AB/TCB fringes though, where generally AB only applies to the home signal and from the section signal). It's a widespread misconception, to be fair to @12LDA28C.
The USA would say "CTC", or Centralised Traffic Control, a system which became widespread from about 1930, with passing loops say every 10 miles, and intermediate signals every mile. These latter are automatic, typically 3-aspect, and allow trains to follow one another on the single line between loops. The dispatcher, like a UK power box, can have say 100 miles of route under control from their console, with a similar illuminated track diagram, and switches for the points, and signals controlling entry to the single line sections.
Until about 1970 the Woking to Basingstoke 4-track line was signalled exactly like this, automatic semaphore signals, with a couple of signalboxes at intermediate stations with crossovers and sidings. Originally lower quadrants, some but not all later became standard BR upper quadrants. Finally replaced with colour lights which effectively did the same thing.
For this exception (where a colour light green can be followed by a red), are all the places where the stop signal is a platform starter at a station where all trains normally stop anyway? I know the Tyne and Wear Metro does this (or at least did when it first opened), but provided a warning board on the approach to each station, and the few non-station stop signals had a separate distant. Also the combination of trainstops and magnetic track brakes ensured that any train passing a red was very quickly brought to a stop.
The equipment used in earlier British Multiple Aspect Signalling schemes was often based on CTC, as seen with the partnership between GEC and General Signal of the US, but used somewhat differently. Also, in the US with most trains being freight (and nobody caring about Amtrak...) it is acceptable to park a train in a loop for some time for several to pass in the other direction. That doesn't really work in the UK context with more and shorter trains and most of them carrying passengers. Allowing multiple trains to follow each other through a single line between loops is therefore less useful, although it may have more value if there is a longer section of double track at at least one end.
I remember seeing a picture of these and a comment that signal engineers were worried that the signals normally showed "off" and would be at risk of icing up in that position, possibly in one of the works of Rolt or Nock. I think each stop signal had a distant beneath it, making it equivalent to three-aspect colour light signalling and meaning a green would never lead to a red. However, there might have been exceptions around the signal boxes. I assume it was replaced by four aspect colour lights (at least on the Fasts) at about half the spacing, to improve the headway.
There are plenty of shots of them in this Youtube video, after about the 10 minute mark when they get past Woking. Being mostly taken from station platforms, the video tends to also include some extra controlled signals for crossovers etc when the local signalbox was switched in. Signals probably about a mile apart, all four tracks together on a gantry. If any signals are showing stop, a train has just passed on that track. They were actually compressed air powered, and the arms moved fairly slowly.
No, they aren't all platform starters.
The North Eastern Railway had a significant installation of automatic semaphore signals (each with stop and distant arms) between Alne and Thirsk, but their arms stood normally in the horizontal position, only clearing when the appropriate track circuit on approach became occupied (and subject to the relevant track circuits beyond being clear, etc.).
So what makes this acceptable at Merseyrail and not anywhere else, where there's no significant difference in visibility, braking etc? Do the signals in question have a distant board or some other warning sign in the absence of a yellow?
It's on the Loop Line, all in tunnel, and presumably reflects London Underground practice. There are some repeaters, but not all stop signals have a repeater in rear.
Since every train on the Liverpool Loop is an EMU with good braking characteristics and the service is intensive, the line was signalled to metro standards as used on the London Underground. Yellow/green repeaters are only provided where a stop signal has insufficient sighting distance for drivers to stop at a red aspect on sight. Every train has tripcock apparatus, and a trainstop is installed at every stop signal. Overlaps lengths are calculated to ensure that a train tripped by a raised trainstop is brought to a stand within the overlap.
Thanks, that makes sense.