Trespass on the railways is quite different to a lot of scenarios described here about supermarkets, etc.
Trespass in most situations is (or at least starts as) as a civil matter. If somebody leaves their front door open and a randomer walks in, that's a civil matter. The owner can use reasonable force to remove the person, but there's not automatically a criminal offence to report.
Trespass on the railways is a criminal offence. The legislation (British Transport Commssion Act 1949) basically just requires a notice to be present. The older legislation (Railway Regulation Act 1840) can be used to enforce trespass rules also, but it requires wilfulness, and also requires a refusal to leave when asked to do so, so not really good at preventing trespass.
If there's no sign saying it's trespass, then it's not an offence.
I'm not sure whether crossing "the line" refers to a painted line on the ground or the railway line, but given how infrequently they are accompanied by a painted line, I'd be inclined to suggest either it means the railway line, or you could at least mount a defence that you believed it referred to the railway line. If the railways wanted to make it clear, they could word it clearer, as the legislation isn't prescriptive in what wording the sign requires. "Passengers must not pass this point.", being a common alternative and arguably much clearer. If you wanted to get pedantic about the 'passenger' point, "No unauthorised access beyond this point.", perhaps.