Need advice from someone who's looked inside various locomotives

Why don't you look at installing it in the tender, like everyone else does? Otherwise you won't have much of a market.

Thanks for replying Dave - I am planning on having a tether so it can go in the tender (which should work for the steam engines). I'm guessing the non-steam engine types wouldn't have tenders (although a boxcar would suffice). 

Have pretty much all of the solutions you've seen involve putting the board in a separate car?  The tender for my 8602 supports 3.5"x1.9"x.75".

Thanks for your reply Dave. I'm pretty excited about this project so I appreciate the help.

I've been inside a fair number of Lionel, K-line, and MTH locos.   I can't imagine you will find many, if any, steamers with that much room in them: your stuff will have to go in the tender.  Another reason for the tender is that many of these steamers have metal bodies and signal penetration could be an issue if you are using radio/wireless/whatever.  Plastic tenders and diesel bodies present no problem there. 

I've taken apart my share of diesels, too.  Most have a good deal room inside than a diesel, but that board will probably present a challenge sometimes, I'm not sure it would fit in a BEEP, which frankly would be a reasonable market you might get (I'm not sure it  I'd check).  Splitting it into two parts that you could stack or put on opposite ends of an otherwise tight, two-motor diesel, etc., might broaden your market. 

Thanks so much Lee. This is great information. As I get this thing working I will report back and let you know how its coming.  Thanks so much for the help.