If we're talking about the final nail in the coffin than I'd guess A. I'm fairly certain that Timur the Lame didn't invade Spain. Invasion was more of the Mongol's style than putting a figurehead on any throne. Correct me if I'm wrong based on what your resources say.
Answer: It indicates that wagon consisting the letter X is the last wagon of the train. It easily alerts the railway officials so that they can save from the accident.
