A lot of beginner mappers have problem with func_tracktrain. I found a lot of questions about that entity, how it works, why it's not working, how to set it up e.t.c. So I decided to make a tutorial about it in my Sven Co-op mapping site. Check it here:

http://sites.google.com/site/svenmanor/tutorials/trains

Keep in mind that it may have some spelling mistakes.