A very fast train of proper length L0 rushes through a station which has a platform of length L (< L0). What must be its speed v such that the back of the train is opposite one end of the platform at exactly the same instant as the front of the train is opposite the other end, according to an observer on the platform?

According to this observer, two porters standing at either end of the platform (distance L apart) kick the train simultaneously, thereby making dents in it. When the train stops, the dents are at a distance L0 apart. How is the difference between L and L0 explained by

(a) The observer on the platform, and

(b) An observer travelling in the train?