London Underground to retire ageing 92-year-old signal box

Very old, very complex.

The "subsurface lines" (Circle, District, Hammersmith and City and Metropolitan) are incredibly old as this article points out and failure prone. It is slowly being replaced with automatic train operation which should be a lot more reliable.

Even on other automaticac train operation based signalled lines like central, northern, Victoria and jubilee you still get the occasional signal fault. Problem is if any of the hardware fails the whole system shuts down as a failsafe.

"Signal failure" encompasses a broader variety of problems than most people probably realise.

These are "fail safe" systems, so for example suppose that a mechanical sensor that is supposed to detect that the points have locked firmly into place so that a train passing over them can't possibly derail has been jammed by a piece of gravel - the system won't go "Huh, that's probably fine though, everything else seems OK" it will mark that junction as unsafe to move any traffic over because something is wrong with the points.

Perhaps a track worker can go examine the points machine, discover a cover wasn't fitted correctly after maintenance allowing gravel to slip into the sensor, remove the gravel, replace the cover and it's back to normal. But if that took them 15 minutes and we're running trains every 90 seconds that's 10 trains out of position, good chance you'll see "Signal failure" reported as the reason trains on that line or a considerable section of the line are delayed for an hour or so later.

There is a commonly heard 'waiting for the signal' if the train is held up, e.g. before a station. You can conflate that into thinking the signalling is the problem when it can be just doing its job. For example if the police have been called to a train and it is held in the station then the signal on the line will stay red and trains will be held up by the signal. 'Just waiting for the signal' is easy on the ears compared to 'the police have been called along with a team of paramedics to deal with a group of drunken crack-heads on the train currently at the station'.

It is the same with 'signalling system failure', there can be good reasons why it has 'failed'. If someone has stolen the wiring or some rodents chewed through the insulation then has it 'failed'? If a train is in the wrong place for some reason then that still means a 'signalling system failure'. Even if this cause has been determined then passengers don't need to know the full details.

There are other euphemisms used for when someone ends their life on the rails, normally you get told that the transport police are attending the scene.

Well, you've got hundreds and hundreds of miles of buried cabling, for a start.

Whats amazing about any of these historically maintained subways is not how high their failure rates are, but how low

Ask any New Yorker, for a comparison. I'm told that from a possible recent high of the early 2000s it has descended into an outer circle of hell.

You have limited maintenance windows to do even routine work (point greasing, cable patching) and almost no window to do complete overhaul.

(uncle worked on Swiss Cottage Tube during WWII)