Demonstrate the correct use of articles and prepositions:They had been waiting at the departure lounge for almost three hours when they finally got _ the train.

Explanation

In the sentence "They had been waiting at the departure lounge for almost three hours when they finally got on the train," the preposition "on" is used to indicate the mode of transportation. When you use the verb "get on," it specifically refers to entering a mode of transportation such as a train, bus, or plane. You "get on" a train because you physically board it.