Rapidly sketched, I have 4:10PM
Let's say distance b/w 2 cities is 300km
Speed of A(fast train) = 100km/h
Speed of B(slow train) = approx 66km/h since it completes 300km in 4.5 hours.
When train A leaves, train B has done 1.5 hours thus 100km. There is now only 200km b/w A and B
Time = Distance/Speed
The sum of their speed: 166km/h
Time = 200/166 = 1 1/6 hour --> 1h10 min
Add it to time at which A left and you get 4:10PM which is time at which they cross. Lots of words for something that can be done within 2 min.