They're working on it, and have been for a while now. The main problem is its just a massive amount of work. They had to develop a fail safe system almost from scratch (Which also has to be interoperable between railroads). And then they have to implement it, and since it requires installing new hardware (antennas, different processors, etc), every couple of miles along the track it is a time consuming process. And then there's all the backoffice infrastructure to manage it, and the railroad-grade hardware to be installed in every locomotive. BNSF alone has 34,000 miles of track, UP a similar amount.

It'll be really neat when its done, and its in operation on some subdivisions, its only a matter of time.

Source: Worked on PTC for BNSF.