Have been working on the same issue:
http://www.picbasic.co.uk/forum/showthread.php?t=9559 might get you started.
I am part way through getting it to work using DT's Int scheme.

The system uses reversing DC on the wires to send data. The length of the reversal is correlated to the data.

I am using the DCC spec as a learning tool, but my uses are different, so I'm changing it a bit. I'm also using auto-detect for polarity into the USART. It is going to require a part that has a selectable comm polarity. A couple of examples are 18F2420 and 18F2525. Look at BAUDCON.4 (TXCKP) and BAUDCON.5 (RXDTP).
Cool once I found out about it.

I am hoping that I can post the results here, but so far, still banging my head like Mister-e's icon!

Bo