In real life, loops joining a mainline are increasingly protected with derail switches. The purpose isn't really to catch speedy runaways (they'll probably destroy stuff and foul the main anyway - except where the shunt ahead is really long, uphill, turning well away from the main) but misjudged brake distances. Example: The current view is correct, however when it's still red, the switch ahead should also be set to diverge. TSC by default sets train path as far as possible, so these protective solutions only act as tiny headshunts. I suppose when DTG built these routes they also thought about this, maybe scriptable even. So just adding a reminder that it would be nice in whichever future iteration.
I'm afraid this would need a massive overhaul of the dispatching and signalling systems, TS knows nothing of "Flankenschutz" concept and signalling green only after a predefined set of points (= Fahrstrasse) is in correct state - TS only checks for a single path (can the next destination be reached through clear blocks?). Zusi has this integrated, but I doubt a general purpose simulator like TS will be enhanced to these rules, and for the vast majority of players this will not be a matter anyway. As tests have shown, changing anything in the dispatcher / signalling code will lead to a large amount of scenarios being broken. So preservation is more important than innovation for our TS. The community had to make the "british" RailWorks fit for german signalling, like Schuster did. The first thing was the Hp0 trigger to make things a bit more realistic, and the GermanSignalScriptsv1.4 patch (essential, available from VirtualRailroads.de) fixed the biggest issues (wrong distant signal behaviour) in the Kuju and Aerosoft signals. TL;DR: TS is not laid out to check if any path could theoretically be crossed by another train ignoring signals, it just assumes (as it is a game) that signal aspects are being followed. In real life, these precautions to protect a signalled path (in your example, the mainline) by all means possible is necessary, as lives could be in danger.
It's also a UK thing, oft spotlighted by Don Coffey. From a signalling perspective it is okay. I understand it's not trivial, maybe neither too bad, though. (basically a casual read receipt to acknowledge your response )
