Place 2 trains, one on A->B and other on B->C, stop them at the station and then release them at the same time (you can do it on each train info window). Check them and when one of them (hopefully A-B since its shorter) is nearing station, stop them again. Then you will place a signal on the spot the train that has not reached the station stopped (B-C).
It wont be 100% perfect, but it will provide a “close approximation”, given both trains run at same speed and you dont have huge slow-down areas (close sharp turns or huge elevations) on any given track.