Complex sentence: Since the painted letters POW on the top of the train are covered by thick snow, the Mustangs fire on the train, to save themselves, the POWs leave the train and spell POW with their bodies and prevent further strafing.
Split into simpler sentences: Since the painted letters POW on the top of the train are covered by thick snow, the Mustangs fire on the train. To save themselves, the POWs leave the train and spell POW with their bodies and prevent further strafing.