Hundreds of London Underground's automated trains were cancelled last year thanks to automated train operation system SNAFUs, blowing a hole in claims that replacing bolshy staff with computers is the best way to prevent delays.

Service-busting failures included the Jubilee and Northern Lines' main vehicle control computers (VCCs) crashing, which causes up to 20 trains to grind to a halt when one of the machines goes down, paralysing large sections of each line.

Other causes of the 756 automatic train control system failures reported by London Underground in the 12 months between March 2015 and March this year included: on-train computer failures; Central Line trains' on-board Automatic Train Protection (ATP) systems failing in service; and the Victoria Line's relatively new signalling systems putting their feet up on the racks and sparking up a digital cigarette.

The Central Line ATP system tells the train's on-board computers whether it is allowed to accelerate or let the train coast, or brake it to a halt. In the event the ATP fails, the train automatically stops itself. A detailed explanation of it can be found here.

A strong argument in favour of computerisation, however, is that the worst delay caused to trains in service as a result of these types of failure was 30 minutes long – with the mean average delay time across all 700+ automated system failures being just five minutes.

Compare that to the delays when Network Rail gets it wrong on main line railways. Holdups of hours are common, as commuters on Southern Railways will know all too well.

The Northern Line's Seltrac S40 system allows the 1995 Stock trains on the line to determine their position to within 1.2 centimetres. Down on the rails themselves are inductive loops which cross over every 25 metres. These can be seen as a pair of thin red cables with one running next to each rail; especially at stations, where the cable switchover points are easily seen. Trains are fitted with antennas to detect these points – but a significant number of antenna problems caused trains on the Central, Jubilee and Northern lines to be taken out of service.

In contrast, manually-driven lines such as the Bakerloo or the Circle would not have had such problems with on-board location detection systems. They rely on the Mk.I driver's eyeball for detecting the train's position, receiving information about the line ahead and observing signals.

With that said, neither the automated Jubilee nor Northern Lines suffered a single Signal Passed At Danger (SPAD) incident, which is the railway version of driving through a red traffic light. Of the 440 Category A SPADs reported on the Tube during the year, 173 occurred on the manually-driven Piccadilly Line* – closely followed by 113 on the District Line. Somehow, ten took place on the automated Central Line as well, but these were mostly in low-speed depot areas. A total of 14 SPADs happened in depots.

SPADs aside, though, what is concerning is the reliability of London Underground's computerised systems. Automatic Train Control (ATC) equipment on the Central Line's 1992 Stock trains failed 93 times with “no defect found” after drivers and engineers alike examined the offending machines. A hidden, recurring fault is often the most difficult to diagnose and cure – and may be indicative of a deeper problem.

As London Underground prepares to change the so-called sub-surface lines – the Metropolitan, District, Circle and Hammersmith & City Lines – over to automated signalling systems in the coming years, perhaps they ought to look more closely at the reliability and suitability of the systems they're choosing. Though unattended train operation might be one for the back burner.

All said and done, no matter how many “sick” days a unionised driver might take off over the holidays, at least his fault diagnosis, train position detection, central processing and fault recording systems are always operational. Heck, he's even reasonably weatherproof, too – unlike 81 of the Northern Line's delicate ATO systems were last year. ®

Bootnote

* Perhaps a reader knows why Arnos Grove is so prone to SPADs? 29 Category A SPADs occurred there in the 12 months from March 2015, the highest concentration in any one Tube location over the year. Particularly problematic were signals A902, A904, PJ320 and PJ350, with each having been SPADded four or five times. If you know more about it, do let us know in the comments section.