In relation to all other points of interest, which are themselves all moving.

It’s not really relevant for rail, no, but not because of inaccuracy and drift, but because the trains are on fixed paths already. Inertial navigation and dead reckoning are accurate enough to get from station to station, and each station can have local markers, even something as simple as a reflector at the end of the platform.

But they’re not developing it just for rail. It would be incredibly valuable for submarines and mining, for example.