by PolyesterMafia » Sat Feb 15, 2014 7:04 pm
Generally speaking, without knowing the specifics of your audio control, the start sound should be be a Triggered Oneshot and should play from .1 (you just started pressing the spacebar or pulling the horn handle) to about .9 (full activation). Your loop should be set up to play from .9 to 1, meaning that as long as you are holding that control in the on position, the loop will play.
For more exact info, use RW_Tools to look at one of the horn audio controls for one of the locomotives RSC has already released.