I have an idea that whenever passengers sit at a table, there could be a random item or items at a table. An example could be a laptop, a bag on the seat or under the table, or food. Suitcases could even be in the overhead storage, It wouldn't be too much to add to the game, but it would add realisim as you walk through the carriges of a train.
Or at least make sure the passengers take them when they leave the train, otherwise they'll have to implement 'suspicious device' security protocols.