But the problem described in the article seems rather unique to NYC and one has to ask how other subway systems manage just fine without artificial slowdowns.
I'm pretty sure the person you're replying to was saying that the original speedometer mechanism in their 1955 car is accurate to within 2-3 km/h of the speed reported by GPS, and thus suggesting that this pre-GPS mechanism ought suffice for the subway / be better than whatever they currently use.
You can simulate the GPS if you wanted to, drop cables down to pipe through the real signal, or simply do very basic positioning with custom radios underground.
OP is correct. We have significantly better techniques.