CP and UP started in I believe 1861. Until they built the first transcontinental line (completed 1867) there really wasn't much going on trainwise in the west, at least between the Mississippi and California/west coast.  For example, the reason for the great cattle drives of the 1870's was that from Texas the nearest railroad line was in Kansas, they had to walk the cattle that far to ship them to Chicago or wherever to the meat packing plants.

Are you 100% sure both sets came with Bachmann EZ Track?? Atlas makes something very similar, if one of the sets was Atlas and one Bachmann and you mixed the two up it wouldn't fit correctly even if both were 18"R curves.

Well, any of the "plug and play" decoders (Digitrax, NCE, TCS etc.) will work. If you're new to DCC, I would suggest using TCS (Train Control Systems) because (in my opinion anyway) their instructions for setting the CV's are much clearer than the other decoders I've used. Digitrax for example gives you a booklet with their decoders that covers a few CV's, then tells you to buy or download one of their books of instructions to do the others. TCS's booklet covers ALL the CV's and what to enter to get the effect you desire.

Anything that would get dirty from coal and coal smoke, a good weathering item is powdered charcoal. You can get it at any good art supply store. I like to brush it on pretty thick, then use a paper towel to lightly remove some, leaving the black in the cracks and crevices and slightly toning down the colors overall. Then you can spray it with any flat finish, it doesn't dissolve the way chalk weathering does.