Everything had been normal; well, as normal as life got on a time-travelling inter-dimensional train. John had worked in the infirmary, ate dinner with Sherlock and Oskar- some hideous vegan dish that Sherlock had scraped together from the remains of food in the cupboards, and he and his partner had gone to sleep in their train cabin, as usual.
A loud screeching sound woke him with a fright- he gasped, blinking into the sunlight for a few seconds... the sunlight? He should have been looking at the ceiling of the train, not sky. He sat up far too quickly, and rubbed at his eyes, terribly confused. "Sherlock?" he called out, hoarsely.
"Oh, God! Sherlock?!" he leapt to his feet, as the reality of the situation started to sink in. He wasn't on the train. He hadn't been sent home. He was... somewhere new? He looked down, taking in his engagement ring, and feeling terribly glad that he had worn some pyjamas the bed the night before.
And then he realised what the noise had been- and John Watson was just the sort of man who ran towards danger.
There were people screaming, bleeding- some dead, some as good as dead- some who didn't appear to have been on board at all. John made his way straight up to the wreckage, carefully stepping over pieces of aeroplane despite his bare feet. "Hello? Is anyone alive in there?"