What is the difference between the following sentences?

When you say "there was a fire" in a home or place, it usually means that the fire got out of control and damaged the structure and hurt people. Saying there was "fire on the train" isn't as precise. It could mean that they had small cooking fires or candles on the train, or it could mean that there was fire painted on the outside of the train. If you say there was "a fire" on a train, most people will ume you mean that fire broke out and burned the train in a bad way.