The train was one of those spots that were kind of between the muggle world and the wizarding world. They weren't at Hogwarts, but underage magic wasn't considered forbidden on the train. Unless someone was injured, or they complained, they were safe.

Shoving her back to the back of the compartment by the window, she turns to the door, considering.

She gives a flick of her wand, the lock slamming into place, then she dropped the tip and there was a clunking sound. "Oops, lock is broken," she says, collapsing on the bench seat. "What has happened, Harry? Could we really be back to year one?"