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Comment by matthewmacleod | Hacker News Reader

You're right that track circuits are simple and relatively reliable, but they are also inefficient. Having to retain sufficient safety margins while being unaware of the real location of trains means that there is excess "dead space" in each signalling block that could be used to drive more trains over the existing infrastructure. It's what happens with systems like the DLR in London, which uses an advanced moving block system.

There is a tradeoff of course, but it's not at all clear that the simplicity of track circuits outweighs increased utilisation of the track.

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Few railroads run trains that tightly spaced. The London Underground and the NYC subways do have that problem, and they've both been cautious about installing newer train control systems. The London Underground's Victoria Line has a modern system, and it's been troublesome.[1]

The enthusiasm for GPS-based systems comes from low-traffic railroads with a lot of track per train. Their maintenance cost for trackside equipment is high for the amount of traffic.

[1] https://www.theregister.co.uk/2016/08/09/london_underground_...

But, as that article says, the lines with automatic control didn't have any trains passing red signals (except in a depot, presumably under manual control). The manual lines had hundreds.

London has at least some ATC on about half the lines, I don't think that's cautious.

From the article you cited:

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.

That is amazing. Bear in mind that the Victoria Line runs over 30 trains per hour in the peak! I don't feel that "troublesome" is an appropriate description.

In the UK at least, a large number of railway routes really are constrained by the safety margins required for trains. In the late 90s, when the West Coast Main Line was upgraded, there was an abortive attempt to implement a modern moving-block signalling system – but it was predictably too difficult for such a complex line.

I don't doubt that you're right about the incentives being different on low-traffic lines though. It does demonstrate that there are various benefits to more automation that need to be traded off against the complexity, cost, and rust of failures.

SF's Muni also uses an advanced, communications-based moving block system. It too has been troublesome. OTOH when it (and the employees) works, it's amazing to see the trains move through the station maybe 15-20 seconds behind the previous train.

So have tighter blocks more often.

A block is a discretization of the continuous track, and in track-space, your absolute "GPS" location isn't relevant -- just where you are on the track.

The on-vehicle location detector ("GPS") is essentially a cost-effective alternative to having infinitesimally small, fixed blocks.

The benefits are equivalent: the tradeoff is between whether you want to spend money on maintaining fixed trackside infrastructure, or more advanced equipment inside on-track vehicles.

Having more and tighter blocks is a distraction, it requires significant maintenance of way (MOW) resources and track time access which is limited to begin with in systems like New York.

The real solution is deploying moving block signals based on Communications-based train control (CBTC) which uses passive rfid balise's placed at known intervals and highly precise tachometers and odometers on the train. This allows any maintenance that needs to be conducted to be done offline, simply drive the train to the dedicated maintenance facility. With CBTC the only item needed to inspect on the right of way is radio system which can be done safely from the cat walk, ie. no need for track access or flagging.

This is what has been done for the Canarsie and Flushing Lines (except they also tie into the existing block signalling to support non cbtc compatible trains). CBTC allows for tighter headway's (more tph) and automatic train operation meaning reducing employe costs (trains drive themselves).

GPS is just a big distraction and should be avoided for mass transit and reserved basically for dark territory PTC.

The block system doesn't give control of the train's speed. They still allow an accident like the one in Philadelphia -- overspeed around a tight curve.

The modern systems generally prevent this. See [1] for an exception -- the engineers forgot to install the limiting markers at the very end of the high speed line, allowing the train to overspeed into the normal speed line.

[1] https://en.wikipedia.org/wiki/Santiago_de_Compostela_derailm...

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