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...
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.