Right about now, Neville should come in, looking for Trevor, Harry thought to himself. Just a few seconds later, his thoughts were validated.

"Has anyone seen a toad?" Neville asked nervously, wringing his hands.

Harry smiled at the young man who would become one of his best friends in the future. "No, sorry. I haven't."

Neville became even more distraught. "I've lost him again! He keeps getting away from me!"

Harry pulled out his new wand, happy for a chance to help one of his friends. "Here, I'll help. What's his name?"

Neville looked at him askance. "Trevor," he said after a pause.

"Accio Trevor!" Harry cried.

A loud thumping noise was suddenly heard coming closer. Then a boy looking to be in third- or fourth-year appeared, having been dragged by an unseen force towards the room.

"What in the name of Merlin is going on here!" the unknown boy screeched.

Harry paled. "Uhm, your name wouldn't happen to be Trevor, would it?"

And that, my friends, is why you cannot summon Trevor on the train to Hogwarts.