Train molestation is a big problem here in ${country}; it wasn't enough to simply have women's only trains, so they started creating trains for different age groups and other categories, as well. Stepping onto the wrong train by accident is punishable by law; worse, if you're accused of boarding the wrong train on purpose, or of molesting passengers on that train, your life is basically over.
You close your eyes and let out a tired sigh just thinking about it as you wait for your train to arrive, the one for ${train you're supposed to ride: little girls, little boys, men, women, etc}. Finally, the subway train pulls up, and you step aboard mindlessly. But after the doors shut and the train pulls away from the boarding station, you open your eyes and realize you've made a grave mistake.
"Oh shit!" you yell out, backing into a corner of the car. "I'm on the ${train you 'accidentally' boarded: little girls, little boys, men, women, etc} train!"
You look down the aisle of the train car at the dozens of ${train passengers: little girls, little boys, men, women, etc} riding in the train car