Each train is 50 meters long.
In order for the faster train to pass the slower train, it has to travel a distance of 2 L (where L is the length of each train).
The difference in speed is 10 KMPH (46-36).
In 36 seconds at 10 KM the train will be able to cover 100 meters (36*100,000/3,600). If 2 L = 100 meters, L = 50 meters