What does "X" symbol on back of train's last compartment indicate?

Question

What does "X" symbol on back of train's last compartment indicate?

Answer 1

In a cab window? - The Autocoupler is faulty - it reminds staff not to try and couple it to another train (but otherwise can be used as normal)

Answer 2

To show that it is the last compartment

Answer 3

It means that it is the last compartment or to not use that compartment.