You can use a single/double slip on a main line, but I wouldn't. Too many issues of cars/engines picking points. It may save you a little space, but slip switches are a maintenance headache a lot of times.
You will see slips on trackwork leading into a big station just to save space and to be able to access any platform from any track. Usually slips are crossed at low/slow speed.
I cannot remember seeing single/double slips out on a working main line.
DIESELS?!?!?! We don't need NO stinkin' Diesels!!!!
The INDIANA RAILWAY.......Still 100% Steam Powered in 2014
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