I was responding to the comment that there wasn't a sensor available that would work.

I worked on a system that sat in the railroad right-of-way and by necessity had to detect an oncoming train from at least half a mile away due to our response time. We were aware that the railroad already had the data on where the trains were, but for RR safety, there was no way that we could get access to that, so we had to place our own sensor next to the track.

You raise an interesting point that if you're near a switching point, you need to know the point's position to determine if there's a potential collision. For my system, it was placed far enough away so that was not an issue: if we detected a train, we knew it was coming our way.