Welcome to Trainz, Tasiyici!
There are different ways to remove derailed carriages from your rails depending upon which version of Trainz you are using. For TRS2004 through TRS2006, there are rerail portals and train remover commands that you can add to your session. I believe these may also work in TS2009 and above, but I am not sure. In TS2010, or at least it appears to be included with this version, is a session command that removes derailed trains from your route after a specific amount of time. I'm not aware if this was available built-in to TS2009.
In general trains will derail if your points are not set properly, or if there's a break in the track such as at a spline point, a connection to a road crossing, or some other fixed-track item such as a station or interactive industry. These derailments will occur no matter what your realism settings are.
John
