Bob and Tim, in answer to your question regarding harnesses:
9-pin is the most commonly used plug in the States. 21-pin is popular overseas and is catching on in the states, while some manufacturers still use 8-pin. Should you run across one of these locomotives I head over to TCS to pick up the harness I need:
http://www.tcsdcc.com/Customer_Content/Products/Harnesses/harnesses.html. Pick up the needed adapter harness from TCS, then plug in the Railpro module. If the trend continues towards other plug designs I'm sure that Tim will start making and selling harnesses to plug into the Railpro module.
You may find that in some cases there isn't enough space inside the locomotive shell to fit everything that you want (module, multiple speakers, resistors, wiring, etc). That's when you take a big step and take out the factory electronics and hardwire in the Railpro module. More work and takes some patience, but allows you much more room to work with and greater electical reliability. This is how I do all my installs now, just because I like it. That being said, in most cases you can just pop the shell off, plug in the module, shell back on and go to town:
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Regardless of how you install the module you will certainly enjoy Railpro! Welcome to the group!
-Kevin