Train A and Train B meets somewhere between NY and Dallas. This means the miles covered by Train A and Train B were equal during that point.
Also, we know that Train A travelled for 3 hours before Train B started.
Distance of Train A + Distance of Train A before Train B starts = Distance of Train B 60t + 60(3) = 90tt = 6 hrs
Train B has covered 90 x 6 = 540 miles when it was met by Train C (which covered the rest of the tracks between Dallas and NY)
1260 - 540 = 720 miles
Train C also started the same time as B. R = 720/6 = 120 mph
Impossible is nothing to God.