I have 2 70 tonners, a 44 tonner and a Spectrum 2-8-0 steam.  Any idea which Digitrax sound decoders to install?

Just go one.  DCC on board.  How and what do I need to do in order to add sound decoder?  And which decoder?

Any recommendations for a Bachmann GP 18, made in Yugoslavia?  Just picked it up from eBay, NIB.  Looks like lots of room for decoder. I'm a beginner and have no idea which decoder to install.

Is the center to center distance different than the 2" parallel tracks?

It has 4 portholes and a metal frame.  The dogbone bearing on the front truck, that attaches to the frame, is broken.  What would be a good replacement truck?

I have an old Santa Fe Super Chief, both sets of wheels are driven.  1: Is it an F7, F9, FT??  2:  The front drive is kaput so I need to buy  a new truck, but since I don't know what I have I am at a lose.  The closest I can figure out, is it looks like a power truck for the GP30.  We're just getting started with a new club and road.  Neither one of us know a lot about HO,  so any assistance will be appreciated.  Oh yes, we are located in the middle of Mississippi and there are now train stores closer than 125 miles away.

Thank,
Larry