On the tube, you're more likely to be sexually assaulted in the rush hour, as the sheer number of people makes it much easier for the abusers to avoid detection.
But really we're talking about perceptions of safety, rather than the reality. And the fact that the guards on many Northern trains lock themselves in the back cab at night makes passengers feel more vulnerable, especially if it's a rowdy train.
That's not to blame the guards for doing it- I'd probably do the same- but rowdy passengers and no staff makes one feel more vulnerable. I've felt like that on the late Airedale Line trains out of Leeds, especially on a Friday night and especially when I'm sober.