I'm not sure specifically as I haven't read that particular fanfic. But it might be about a character named Harry who has some sort of experience related to three trains. It could be an adventure where the trains play a major role in training Harry, perhaps in a magical or extraordinary world.
It could be Sirius Black. He has a lot of experience in the wizarding world and, despite being on the run, he could find a way to communicate with Harry secretly. He could teach Harry about dark magic defense, which would be very useful in the tournament.
For sure Harry is there. But there could be new characters introduced. Maybe a foreign wizard who has come to Hogwarts for the summer and joins Harry in his training. The Weasley family could also be involved. Mrs. Weasley might be sending care packages to Harry during his training. And Mr. Weasley could be sharing some of his knowledge about the Ministry of Magic's regulations regarding certain types of training that Harry is doing.
Maybe Harry could meet new friends while training. He might discover new magical abilities related to his training. Or he could face some challenges that test his limits.
There could also be new characters introduced in the fanfic. Maybe a new mentor figure who guides Harry in his training. This character could have unique magical abilities and knowledge that they pass on to Harry. Another possibility is an antagonist who tries to disrupt Harry's training for some reason, adding conflict to the story.
I'm not sure specifically as it depends on the author of the fanfic. But it could include the classic Harry Potter characters like Harry, Hermione, and Ron. They might be on the train for some special reason related to their fight against Voldemort or some other new threat.
One great 'moody trains harry fanfic' could be one where Harry has deep, moody thoughts while on the Hogwarts Express. Maybe he's reflecting on his upcoming challenges at school. For example, he could be worried about dealing with a new dark arts teacher. In such a fanfic, the train journey becomes a time for him to introspect, with the passing scenery outside the window adding to the moody atmosphere.
Harry is the central figure, but there might be some new characters introduced in the fanfic. For example, an old wizard who lives on the outskirts of Hogwarts and has some unique knowledge about magic that Harry discovers. This character could become a sort of mentor, even if from a distance, as Harry trains himself using the knowledge he imparts. Also, there could be antagonists like some students who are jealous of Harry's self - improvement and try to disrupt his training.
One interesting aspect could be the character development. In fanfics, authors often explore different sides of the characters. Maybe in 'harry potter trains hard fanfic', we get to see Harry Potter training in ways that were not shown in the original books, like developing new spells or combat skills. This can give a fresh perspective on his growth as a wizard.