how do i change default switch?

ok so i have a scenario that i use for testing all new or working on engines, thing is on my main line there is a switch by default goes into a siding with a dead end, id like it to be by default stay on the mainline, or maybe by default its on the mainline, but it ends in a dead end, i cant remember now, either way can i have a switch stay in the position i want? im not sure how to make this happen 
