My eldest son does that particular train journey most weeks, thankfully not on that particular day. He was on that route a couple of days later though and said there was a police presence at Cambridge station but not on the train itself.
I've just read an article which says that the train crew member, Samir Zitouni, who was seriously injured trying to protect passengers, regained consciousness briefly and was able to speak to his wife for a short period of time. He 's still got a long journey to recovery but it's a good sign.