Why ‘X’ written on last compartment of train?

If a train passes a station and the Station Master notices that the last compartment doesn’t have a big letter ‘X’ then he becomes sure that the train is in trouble.

Why ‘X’ written on last compartment of train?

You must have noticed a big ‘X’ mark on the back of the last compartment of all trains. Have you ever thought what does it denote?

Read on to know why.