ehh, I dont think it was real, I believe it was in his head because the noise of the train dies down without it even stopping while Michael is stillin the bathroom but the audience can actually hear it stop once he decided to shoot them when hes back at the table. I think it didnt stop in the bathroom cuz it was showing he still had the option of not shooting them but then once he did make the option to shoot them, the train came to a halting screech showing there was no goin back like someone else jus mentioned. Plus, trains dont come after one another that fast.