I expect they would enter the room immediately but upon seeing a significant weapon, such as a knife/hammer/baseball bat, they would back off. That's what I would be doing. The crew member isn't going to be able to do anything against an armed attacker and I can't imagine they have the training to do so.
The reports on this man don't suggest he was carrying a weapon, and he assaulted both male and female travellers. CS staff I believe have bodycams now though I'm struggling to remember from my more recent journeys if these were ever rolled out. They may not physically stop somebody with a knife, but they'd contact the police who would respond pretty sharpish.
I suspect the reality is more that victims often freeze up in these situations, and that in this man's case some of it was verbal rather than physical, and while still awful, some people brush off verbal remarks.
Whether you would hear goings on between rooms on the sleeper, I'm not convinced really. The person in an adjacent room is usually somebody sleeping, on quite a noisy train, rather than an awake staff member. They do walk through the train every now and then but it's not hard to see that they've just left and in my experience they don't go through particularly often.
It actually in my view reinforces the suggestion of having more beds in the rooms and allowing sharing - you're more likely, in my opinion, to get help for such an incident if there's 3 other people in the room rather than being solo. Though it is a bit unclear from the reports how he gained access to locked rooms in the first place, which is probably something else CS need to look at.