I picked the 11.06 Lackenby to Dalzell service in the timetable. Do what you need to until you stop at location at a red signal just before joining the mainline.
Wait for signal to go green, fully release brakes and then save the game.
Location marker says you are 15 miles away but I get credit for 20-21 miles each run.
Just keep replaying service. No red signals, no passenger trains to get stuck behind