The most I've ran was 5 AI's, 2 in front and 3 in back with no derailments or anything you mentioned. If you want constant traffic, meaning running into red signals and such, just have the trains in front of you maybe 30-60 seconds in front of you and the same behind you I guess.
I'd figure the correct way for this would be:
Route.RunInterval 30; 60; -30; -60