The Midland Mainline DLC that releases tomorrow has a layer for SEHS. I bet this patch for SEHS has the changes required to make the game aware that it has to load the HST from MML and add it to the timetable if it is availabe.
Nothing about SEHS. It only announces, Patch Notes: 17th April, Boston and New York - Trenton Edit: Found it. Fixed a crash on South-eastern High Speed ‘Class 465/9 Introduction’ for PS4 and PS5 players.