UK railway signalling | The basics!

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How do British Railway Semaphore and colour light signals work? Today we take a look into it

[@mrwizzardknight]

Why DO some semaphore signals go up instead of down to show all clear?

[@reliantrailways]

Some signals do go down, but the main reason to go up is to avoid snow / debris forcing the signal down and showing all clear all the time

[@mrwizzardknight]

@@reliantrailways Oh? Makes sense! ;) Thanks! ;) If only it were designed that way then Henry wouldn’t of had his crash pulling the flying kipper

[@clifftonicstudios7469]

Different rail networks also

[@bussesandtrains1218]

safer. if you snap the wire, it goes to danger.

[@gregoryclark8217]

​@@bussesandtrains1218incorrect. Both upper and lower quadrant signals will return to their most restrictive aspect if the control cable snaps. This is basic fail-safe design. This is accomplished by the different placement of the lenses with respect to the pivot. With lower quadrant signals, the lenses and their mounting (quite heavy to ensure counter balance even with snow on the arm) are fully to the right of the pivot. Upper quadrant signals have their lenses mostly underneath the pivot, which reduces their effect on the balance of the signal arm to a minimum.

[@j.a.g1291]

A good video. short, simple, and easy to understand. Some points to make clear tho, as there is some important missing information: With semaphore signals, the distant signal relates to ALL the stop signals controlled by the corresponding signalbox. The distant signal cannot show "clear" unless EVERY stop signal controlled by the signalbox shows proceed. This means that should a driver pass a distant signal at "caution", they MUST expect ALL the signals controlled by the corresponding signalbox to show "danger". The signalboxes have interlocking lever frames, which physically prevent distant signals from being switched to "clear" (also known as "pulling off"). This is why a driver can assume he has a clear route through the area. In practice, there is ALSO a rule that if a signalman working a signalbox can only clear SOME of his signals safely, then he must NOT clear those signals until EITHER the train has been brought to a stand or crawling pace at the signal OR he can clear ALL his signals. This gives the driver even more information. A scenario is the best way to understand this: Imagine a signalbox which controls 4 semaphore signals in one direction: one distant and 3 stop signals Say a train has just passed the distant semaphore signal at "caution". That train is now slowing down, since it knows the next stop signal will be at danger. In fact, the signalman has a clear section in front of his first two stop signals, but there is a train in the section controlled by his third stop signal. This means he cannot clear ALL of his stop signals, which is why the distant is a danger (it physically cant be changed unless all stop signals are showing clear) The signalman STILL cannot clear ALL of his stop signals because that train in his last section, so he now MUST look outside and observe the train coming to a stop or a crawling pace in front of his first stop signal. Only then can he clear that first stop signal and let the train move on. And so, the train passes the recently cleared signal, and MUST expect the next stop signal to be at danger. Now, the train that was preventing the signalman to clear all his signals has exited the section. The signalman can now clear ALL his signals, so he does exactly that. the remaining stop signals controlled by him are now showing "clear". The train that was brought to a stand, however, has only just passed the first stop signal, and is expecting the next signal to be at danger. But when that signal comes into view, it is already displaying "clear". This lets the driver know that the rest of the signals are showing "clear", since he was NOT brought to a stand with the signal at danger BEFORE the signal was cleared: it was already clear. The driver can now accelerate to line speed until he sees the next distant signal (controlled by a DIFFERENT signalbox). In reality, most signalboxes did not controlled this many stop signals, but some of the larger signalboxes covering an extended area did. Also to finish this off, at junctions, you may have multiple signal arms stacked on top of each other. Each arm corresponds to a route. Signals are read top to bottom, left to right. The highest signal arm corresponds to the main route (straight through). The lower arm corresponds to a diverging route: if it is on the left, then the route diverges left. In the case of multiple possible routes diverging left, then the leftmost arm corresponds to the leftmost route.

[@reliantrailways]

Thanks for the extra info!

[@ianburnett7333]

When I was at VSC Clapham junction, lots of signalling points failures, ASE was 'Wrong man for the job'had to be paid off. Replacement narcissistic told by Chairman BR Board that he was to be sacked in 6 months time unless failures dropped. Bought in man from Swanley led a task force team found lots of equipment faults, he reduced failures.

[@reliantrailways]

That's a very interesting story, typical chairman.

[@robnewman6101]

Really Useful Railways.

[@himagainstill]

The yellow shunt signals mean that the horizontal "stop" aspect only applies to movements onto the main line, and can be passed if the points are not set that way. This is a legacy rule, and yellow shunt signals are now only allowed to be installed in like-for-like replacements, they're not allowed in new designs.

[@railwaymechanicalengineer4587]

Your incorrect.

[@EXSKIN]

@@railwaymechanicalengineer4587 Try this one then, from MP12 driver training 1985.This signal can be passed in a direction that when it is cleared does not apply too.

[@ased4837]

The disc signals are called shunting signals and what you call shunting position light signals

[@Fall.Movies]

Good job man, i work on building these signals

[@seanC3i]

Interesting. Just to add, there were two types of semaphore signals, Upper Quadrant and Lower Quadrant signals. Both show a horizonal bar for a stop (or caution aspect in the case of distant signals) but while a lower quadrant signal goes down 45 degrees to show clear, an upper quadrant signal goes up 45 degrees to show the same. If you watched Thomas the Tank Engine, the episode where Henry crashes while pulling the Flying Kipper demonstrates why lower quadrant signals were considered less safe and thus used less as time went on.

[@bussesandtrains1218]

except the western region. nuff said

[@EXSKIN]

Quite simple Western region chose lower Quadrant every other region chose upper. nothing to do with being less safe. And your quoting Thomas the Tank as a signalling guide.

[@j.a.g1291]

when semaphore signals were first introduced, lower quadrant signals were used. However, it was evident that there was a problem: when the wire snaps, gravity pulls the signal arm down around the pivot (since the wire is no longer providing a counter force), and the signal shows a proceed aspect, when in fact it should be a danger aspect. I haven't seen the thomas the tank engine episode you are referring to, but i would assume thats what happened in the episode. This type of failure is very bad. It is referred to as a wrong side failure: where a piece of equipment shows a "false positive" (a signal showing proceed when it should show stop) In signalling (and other safety critical systems), you need to ensure that should a failure occur, the most restrictive information should be shown. This would mean that should the semaphore signal fail (due to a wire snapping), then the signal should default to a danger aspect. This is called a right side failure: where a signal should show proceed, but instead a danger aspect is shown. Early railway companies had to solve this issue, as currently with the lower quadrant semaphore, the very bad wrongside failure could occur. Most railway companies fixed this by introducing the upper quadrant signal. Using the very thing that made the wrong side failure occur, the upper quadrant signals were "failsafe". should the wire snap, gravity (the thing that caused the wrongside failure in the lower quadrant signals) will pull the signal down into the danger aspect. This solution did have a drawback: drivers now needed to be retrained (or at least told) that the new proceed aspect is a raised arm, rather than a low arm. But this was a minor drawback, so most railway companies used this solution, including the vast majority of signals in the uk right now (in fact i dont think there are any lower quadrant semaphores left on the national rail netwrok, but i might be wrong) Great Western Railway (GWR), however, were stubborn. They did not want to adopt this upper quadrant system to make their signal failsafe, so they came up with a different method. GWR installed counterweights on the signal, weights that were heavy enough to pull the signal to danger should a wire snap. When the signalman pulls the lever, they are pulling the counterweight UP, thus allowing the signal to be lowered. Whilst GWR came up with a rather strange and convoluted solution, it worked. should the wire snap, as explained before, the counterweight falls, pulling the signal into the horizontal danger position. GWR made them FAILSAFE, as long as they were installed with counterweights. I can assure you that since GWR installed their method, the lower quadrant signals are no less (or more) safe than upper quadrant signals.

[@treinspotterhugo]

do you guys know the reason why red is on the lowest row its becuz if there is a heavy snow storm red is under so you can see its red and then you know when too stop or else you will not see the red light

[@reliantrailways]

A great insight there!