Ok, I found out how to do it with RSCs video tutorials. You have to angle the siding towards the mainline and lay down a bit of track, then the next section of track, the section that will connect to the mainline, has to be laid as a curve angling away from the mainline. It's a bit hard to explain, here's the video:
http://www.railsimulator.com/rstv.php?Ref=15
It's a little tricky at first, but once you get the hang of it it's very easy.