Let the velocity of the train be v. The relative velocity is (v + 5) km/hr. Converting to m/s: $(v+5) \times 5/18$. The distance is 100m and time is 7.2s. So, $(v+5) \times 5/18 = 100/7.2$. This gives $v+5 = 50$, so $v = 45$ km/hr.