How were the two trains headed eastward that were side-by-side different?

The train Bruno's family was traveling in was spacious and practically empty. The train beside his, traveling in the same direction was packed with people.

The official car with the flags on the front had taken them to a train station, where there were two tracks separated by a wide platform, and on either side a train stood waiting for the passengers to board. Because there were so many soldiers
marching about on the other side, not to mention the fact that there was a long hut belonging to the signalman separating the tracks, Bruno could only make out the crowds of people for a few moments before he and his family boarded a very comfortable train with very few people on it and plenty of empty seats and fresh air when the windows were pulled down. If the trains had been going in different directions, he thought, it wouldn't have seemed so odd, but they weren't; they were both pointed eastwards.