A train running at a certain speed crosses a stationary

A train running at a certain speed crosses a stationary engine in 20 seconds. To find out the speed of the train, which of the following information is necessary ?

1. Only the length of the train
2. Only the length of the engine
3. Either the length of the train or the length of the engine
4. Both the length of the train and the length of the engine

Since the sum of the length of the train and the length of the engine is needed, so both the lengths must be known.