The blanks in this flat can be filled in with two words which differ by two additional letters in the second word. Other than the two extra letters, the order of the letters does not change.

The three desperados galloped on horseback, chasing the train,
Trying to board the _ _ _ _ _ _ _.
But as luck would have it, the law was tipped off,
So they wound up in the _ _ _ _ _ _ _ _ _.

Answers:

CABOOSE
CALABOOSE

Credit:
The Enigma
NPL Magazine
June, 2017
TRAAL BY FIRE (adapted)