That is fascinating, thanks for sharing. I had no idea the NYC subway was being run by a 100 year old electro-mechanical system.
Safe train control seems like it needs a little more sophistication than '60s or '70s tech was able to give. For one, it's common sense that a train that disappears from the system isn't really gone-- but the Metro system couldn't take that into account. Stuff like that should be pretty trivial today.
It was pretty trivial in the 1970s. Require the sensor for an adjacent segment to be engaged before the sensor for an active segment will disengage.
You can't do any computation with modern computers that you couldn't have done with relays in the 1970s, or the 1870s for that matter. Mechanical relays are slower, less reliable and more expensive than transistors... but they do the same thing.
The system was replaced a couple of years ago with a new one, to allow an even higher frequency of service.