Question 829037
You did not actually ask a question but the situation can be analyzed.

The 3PM train is at speed r.  This train travels for time x+3, assuming it still travels after the 6PM train begins its travel.  Meanwhile, after 6PM, the 6 PM train travels for x length of time.


__________________speed______________time______________distance
3pm train_________r__________________x+3_____________(unknown description)
6pm train_________r+54_______________x_______________(unknown description)


You would want a question about some total of time, or some total of distance, or some difference in distance, or some comparison about distance; or some description allowing you to find x or r.  You would make use of the uniform rates idea, {{{Rate*Time=Distance}}}