The link that engineers and conductors use for track allocation can pass the data from the remote to the central receiver then relayed back to the engineer. However, it would take additional hardware and software at the central station. It would be similar to an amateur radio digital repeater.
A 915MHz link is possible but it has two serious problems. First, a Yagi antenna would have to be mounted just above the top of the last car and the engine. With this short height, you have the possibility of the reflected wave being out of phase with the direct wave in a continuously fluctuating manner and getting serious signal dropout. Also, the antennas would have to be of reasonably high gain with an associated narrow beam width. As the train goes around curves, the antennas are not looking at each other.
On the remote unit, consider a small wind generator mounted on the back of the antenna to keep the battery alive.
For reliability, the first option seems more reasonable.