I get the impression that you really don't understand the sheer numbers of people involved. It's not a couple of trespassers at only one location during the journey, it's a couple of idiots like those pictured above at one point, then a group like those videoed at the next bridge, a couple more climbing the fence in the field after that, a family or two messing around at the next footpath crossing and so on and so on for the whole trip - and that's ignoring the people who aren't doing anything wrong but who's behaviour suggests that they should be gently watched in case, in their ignorance, they do put themselves at risk.
You do realise that in much of the country BTP struggle to round up the half-a-dozen officers to police a football train, don't you? There's absolutely no way they could pull together enough officers to arrest the number of people trespassing when Scotsman is around, even if those people docilely cooperated with being arrested. And every arrest means the arresting officer gets to take that arrested person to the nearest police station with a custody suite, which then means there one fewer police officer for the rest of the journey...
The local police are no better off; they'll often have fewer than half-a-dozen effectives at any one time across an entire division, so it's not as if they can call up a few van-loads to assist BTP.