I find it hard to understand why it's so costly. It seems to me that it could have been done with a few tweaks to existing GPS receiver technology. I do understand that it also detects obstructions on the track, but that feature's probably useless anyway. By the time an obstruction is in the field of view, it's likely going to be too late to stop the train anyway.
Rich
For the same reason that certified avionics are so costly. The Federal DoT (Federal Rail Administration is the part of DoT that handles this) requires certification of the system.
GPS would be the obvious solution, but it doesn't work in tunnels or areas where the signal is obstructed.
Here's an example: the Washington Metro System has a fully automatic control system and alerting system. It uses loops on portions of the track and sensors in the cabs to detect the location and speed of the train & takes input from the control center to control the train. It's 1960's technology, but it's certified for the system. It's capable of smooth operation of the train, precise stopping at the stations, and collision avoidance.
In 2009 (8 years ago) there was a crash between 2 Red Line trains because of a failure in the automatic control system. Basically, it failed to sense a stopped train and sent the next one plowing into the stopped one. 8 deaths, 80 injuries. Since then, the automatic train control system has been disabled and the trains have been run manually. Yeah, Metro is poorly managed and lacks funds, but this was a critical part of the safety system. It tells me that they didn't trust it after the crash. Within the last year they have turned on a small part of the ATC system on one line of the system - the rest remains off. That's despite a year-plus long track rehabilitation project.
https://en.wikipedia.org/wiki/June_2009_Washington_Metro_train_collision
As I understand it, the PTC system uses a variety of sensors to detect things like train speeds and impending collisions. It uses radio data sensors and track block sensors to gather data. There are a couple of different systems being installed, so I'd expect someone like Amtrak to have to accommodate multiple systems. I don't know all the details of the systems, but I do know that being a safety system certain certifications are required. It's not cheap.
How much has been spent on ADSB infrastructure for aircraft? And how many planes still don't have it implemented?