He hadn't noticed the students coming in although he should have expected them. When, in his days – the days when he and James and Sirius and Pete would spread out over a compartment together – had teachers ever travelled on the Hogwarts Express? These students – young teens who barely came up to his shoulder – must have been very quiet. He only awoke from their scuffling and falling over each other as the train darkened and chilled in the grip of the Dementors.
He remembered why he had boarded the train. Not to remember how he and James and Sirius and Pete would sit together, share their sandwiches and fire chocolate frogs at each other - the memories flashed past his eyes half formed – but to guard the train. To stop Sirius in his tracks if he needed. To protect James' son.
He heard the five – was it five or six? - students shaking and gasping as the Dementor blocked his path. One of them must have slid onto the floor. He heard the light scuffling of clothes on the floor and irregular thunks as a trainer clad foot caught the bottom of the seat. The smallest girl had shrunk right back into the corner seat, just by him. The cold despair washed over him and he remembered the bite that separated him from true humanity, remembered waiting for the moon to rise and the madness to take his mind, remembered that here there was no James – dead, thirteen years ago, no Sirius – a howling mad traitor and not even little Pete – "blown to bits, they say…all they found was his little finger" at his side. There were only a few children who couldn't stop the creature but with a deep, dispassionate love, Remus loved children. The urge to stand between a youngster and all harm was deeper than any werewolf bite. The effort to cast his Patronus took very nearly more strength than he had – and what a weak, formless thing it was – his despair told him, but it was strong enough to chase the foul thing away from the teens huddled behind him.
Even as the lights flickered on, the students were on the floor, pulling the fallen student up.
"Harry! Harry! Are you alright?"
'W-what?'
He didn't know the other students. In another life – a different world – he would have known the red haired boy and the slight red haired girl – surely his sister -, the tall, round faced boy with the anxious expression and the girl with brown bushy hair, kneeling on the floor. But he would have known Lilly and James' son anywhere. After nearly thirteen years, it was Harry he had defended.