*DO NOT* re-invent the wheel. Making control networks work reliably in a rail environment, over automatic couplers, under extreme environmental conditions, possibly with nearby high-voltage arcing, is a very complicated matter. Use what is already field-proven. Refer to IEEE-1476 for two good candidates, namely Echelon LonWorks and TCN.

I design systems with these networks on real trains in revenue service. I am an active member of the IEEE-P1544 working group. I know what I'm talking about. Don't take this stuff lightly or you could end up getting a bunch of people killed.

I suggest you check out this website for more information and links to professionals that can handle these networks:

http://www.tsd.org/