I felt a bit traumatised last night. As a PP said, the drama of people's lives - particularly women's lives as relates to childbirth, and some of the storylines the series has brought up around abortion, poverty, homosexuality, domestic abuse - was enough and The Crash felt a bit like the time they let an EastEnder's scriptwriter loose on The Archers. Much as CTM seems to exist in a rather perfect bubble where everyone does the right thing/comes round in the end/magically manages to persuade the Council not to flatten Nonatus House for an office block, I don't think it needed a massive disaster.
On top of that, you could see the plot line - train driver with eyesight/blinding headache problems who has a wife about to go into labour, people on a train when they wouldn't usually be ("I'm so glad I decided not to drive...") and everyone being happy - coming like, well, a train coming down the tracks.
As for next week, I suspect Dr Turner will survive, Sister Julienne won't (or she'll be injured enough to take retirement at The Mother House) and Nurse Crane will happen to be in The Carriage of Doom to do some heroic saving. Bloody love Phyllis, she always seems like a good egg.