How do you make a part detect another part touching it? Please explain.

I made a railway crossing, but I have no idea how to make it detect the trains. My idea is to make it detect any parts touching it, then set a boolvalue to true. My problem is that I have no idea how to detect a part touching it. Please explain how to do so. I don't want a script, just an explanation of what parts of the API to use and how.