Well, trains travelling in opposite directions don't travel along the same track and don't 'follow' each other in 15 minutes (the time will depend on the location...), you could just have ignored directions. Or said that there are two trains on 150 km of line (= two parallel tracks). But due to acceleration/deceleration, I didn't calculate with top speed but just took 100 km, which is also a reasonable distance between stations.

_{*Lunatic*, n.
One whose delusions are out of fashion.}