Happy September 1st ✨️ Let us partake in the 100,000th Harry Potter Marathon that we need no sufficient reason to have one, but it's still a good enough reason to do so.
“A scarlet steam engine was waiting next to a platform packed with people. A sign overhead said Hogwarts Express, eleven o’clock. Harry looked behind him and saw a wrought-iron archway where the barrier had been, with the words Platform Nine and Three-Quarters on it. He had done it.”
Okay for the Marauders writers, you're doing like a fic that has a scene in King's Cross itself, it would have been very confusing for the local wizards.
In 1972, the muggles added a big concourse out the front and organised the platform numbers so they ran in order from east to west. I'm not sure exactly what the numbers were prior to this, but considering the order of construction I think they would have been all over the place. But I think it means their platform would not have been call Platform 9 3/4 until then. So at some point in their time at Hogwarts, this became the new name of the Platform.
The 70s was a period of electrification, which various wires going up and around the station. Between 1975-1982, the InterCity Class 125s, which were a mix of diesel and electric were made and started to be see around King's Cross.
In 1976, if any of the Marauders are looking out the window as the train departs, they might notice that the muggles have put up large overheads. By the time its Christmas 1977, there are new fully electric trains on the track!
In 1977, suburban and regional services now terminated at King’s Cross using platforms 9, 10 and 11. Previously this happened over on the York Street side!
So that's a bunch of fun things for the muggle ignorant wizard kids to contend with lol.