Signal Failures Explained

By londonist_alex2 Last edited 144 months ago
Signal Failures Explained

On our way to the Londonist Dungeon this morning, we were unlucky enough to be travelling in the Central Line as it went absolutely tits up. Our friendly and throughly engaged driver told us that the problem was a signal failure at Bethnal Green. Having used the Central Line for nigh on 12 years, we've become used to this kind of bestial shit, but this morning we found ourselves wondering..."What does 'signal failure' actually mean? And why do they happen to the Tube so much?".

It felt time for a little bit of investigation.

London Underground define signal failures as the following...

Where a Train is delayed, followed by a period of through-running at reduced speed affecting subsequent Trains which is due to a signal, points or track circuit Asset Failure or Defect until such signal is fully restored.

If you peruse the LU website, you'll find the following questions & answers...

How do signals work? And what does it mean when there's a signal failure?

The basic purpose of railway signalling is to keep trains a safe distance apart. Railway lines are divided into sections called blocks and only one train is allowed into one block, protected by a signal at its entrance. Low voltage current is supplied to the running rails, which when short-circuited by the passage of a train’s wheels, operate relays that control signals. The average length of each section on the Underground is about 300 metres. To prevent a train from entering into an occupied section, each signal is provided with a mechanical ‘trainstop’ adjacent to the track. This device will apply the brakes of any train, which attempts to run past a stop signal.

Why are there signal failures on the Tube and what causes them?

There are many reasons why signals fail. However, in many cases, the signalling system itself is working normally, but the equipment has detected a problem with the track. Because signals are designed to ‘fail safe’ whenever a fault occurs, signals turn to red and trains stop running.

As the Tube's signalling system uses small electrical currents in the track to detect the movement of passing trains, signal failures sometimes happen when there is a short circuit between the running rails. These short-circuits may occur after heavy rainfall, when puddles of water build-up on the track - particularly on our open air sections of line. Also, with our high frequency of service, the accumulation of iron filings (from the daily wear and tear of trains) across insulated joints between sections of track may also cause problems. There have even been cases of rodents chewing through cables, turning signals to red!

When disruption occurs, the safety and comfort of our customers is our priority at all times. Our line controllers always ensure that any trains which are stuck in tunnel sections are moved into stations as quickly as possible to let people off.

Measures we are taking to reduce signal failures include replacing old track, improving drainage, cleaning block joints at regular intervals and improving the insulation between track sections using a new type of rail joint.

So according to LU, signal failures are just a natural part of life. Like taxes and awful television on ITV. But according to Ken (via the Beeb ), signal failures have increased by almost half between January and April of this year. In response, Metronet said signal failures on its lines fell 4% over six months and Tube Lines said reliability had improved. But they're clearly talking bollocks aren't they?

This morning our tube system felt like the worst in the world. Or are we being too pessimistic? You tell us.

(p.s. check out this new tube route finder . it's rather neat.)

Last Updated 27 June 2006


lobocks indeed. my bosses still don't believe me when I'm late thanks to the tube. Only became they have emigrated outside the M25 and get the hallowed overground.

David Tran

OK, so according to my calculations, a train travelling at 70mph can cover this 300m in 10 seconds, so what happens when the signals fail and the train do not stop ? does that mean you have 10 seconds to, ehr, pray ?


Yes, track circuit failures are a part of life, and I know this because I am a signalling technician - they happen for all the reasons put forward by LUL and more, I can assure you. Reductions in failures can be achieved by better maintenance and better equipment (some types pf track are more reliable than others) but obviously this depends on the company involved - and not being a Metronet employee, I wouldn't like to comment on that one... and to David Tran, there's a little more to a signalling interlocking than having 10 seconds to pray.


This is bullshit. If signal failures are a part of like for our underground network, then we need a new underground network, because ours clearly sux. everyday i look at the tubes to see which line has a signal failure and everyday theres at least one that has minor delays, severe delays or part closure. Usually its at least 2 or three lines. today the central line (which I am unfortunate to live on) has part closure due to planned engineering (like it does every other weekend) and so i am forced to get a bus to Walthamstow and use the victoria line. But wait! theres been a signal failure and severe delays occurring to the whole line. This happens to me almost every day! Theres obviously something wrong with our tubes and it needs to be worked out.


The real reason is the people in charge and the maintanance staff are actually thick idiots. They need to attract more brainy people to the jobs. There is a reason signal failures NEVER happen in other countries like Hong Kong and Japan.

Grrrrr !!!

The reason the tubes dont run well is due to the monopoly that LU has on transport. They get to charge the maximum fares for the worst service and there is zero competition as an incentive for them to do better. Throw into the mix the low IQ individuals they hire, that are protected by the unions and a benign and accepting populace, that would gladly accept a whack on the head upon boarding trains, if it were included in the T's & C's... In any other country this wouldnt happen...


Overground between richmond, clapham junction and willesden junction have problem every day !! I call it the Bermuda triangle... lol the worst is that i don't get pay 1 hour if I'm late 20 min + to work...and on top of that i have to pay extension on my travel card every time they have problems... bastards !!!! i give them good money every month and i get the worst service...the government should force them to stop charging full price on monthly and weekly travelcard when they have a certain amount of hours of disruption !!!!

LU signals

as a signals tester for LU I can assure you we are not all the thick fools you say we are in the comments. There are different professions and as a signals tester I am highly qualified. But regarding the safety, there are signals for signals for signals in basic terms to make sure the public are always safe. Regarding the worse underground in the world comments we are the oldest. no one assumed so many people would use the underground network when it was first built. The rails become worn, the cables get snapped, the bulds blow. Or it can just be as simple as a fuse, but we have to come from parts of London, down the track and find this one in a thousand things that could of gone wrong. So although it may seem long delays, we try to fix things as fast as possible.
PS reference the weekend works, LU see it as a less busier time less profitable times to do the work (nights/weekends). this is why most places have works being done on weekends :D

Marcos J. Rodríguez

We could even trust in what they say, but the question is: Why this happen in London Underground so frequently whereas in another tube networks is once a month as in Madrid is?


I've once been told that "signal failure" is just a euphemism for "person under a train".