Bill, if you are doing a deadrail install with Railpro in a steam locomotive, it might be beneficial to buy DCC-ready and plug the LM into the supplied harness (like Tim described). If that takes up too much space, remove the factory board and hardwire the wires to a new 9 pin harness for the LM to plug into. Space might not be quite as much of an issue in a steam tender and it is in a typical hood unit. However, you'll find that space goes quickly once you start adding in the necessities....battery, LM, wiring, speakers, lights, etc.
On my own locomotives I prefer to completely remove all factory electronics and hardwire the LM in the best place possible. This leaves me with maximum space. So much easier to do an install when there is ample space.
Depending on the battery you will be using, you'll need a voltage step-up board. I used a 3.7 volt battery and used a tiny Pololu board to bring it up to 12 V. Worked like a charm.
-Kevin