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