40.6 miles/hour (x hours) + 40.6 miles/hour (2 hour head start) = 50.2 miles/hour (x hours)

x ~ 8.45 hours

actually it would take train B less than 8.45 hours unless train A is infintessimally small. one would have to know the length of train A to know at what point train B would collide with the back of train A...

also, how is it possible that these trains instanaeously leave the same station at a constant velocity? that is a pretty awesome feat.

h20