Well from what I've ascertained (just from reading) you download the actual base map holders from the DLS, you very carefully copy a image from Google Earth or some other map and and it has to exactly meet certain measurements. Then you have to combine the two in CCP assigning a different kuid to each, then you have to put them all together in Trainz.
Now if I left out a step I'm sorry.
However there is an alternative. TransDem works very nicely, you can select where you want to create a map of, download DEM's, then either overlay Tiger vector data or any polyline and TransDem will pull raster maps, Google or Yahoo maps or ortho photos or all three along that route or polyline.
Once you have all that data it will then create UTM tiles which are the same as the base map and automatically create different kuids, and place them on the map under the surface slightly where they can be seen in high-resolution using wireframe mode, and it does it for you automatically.
And a added bonus, it will take your vector data and actually create track or road or rivers or whatever right into the map for you all automatically.
Now the difference between the two is one is free and requires a bit of work, and the other is a small one-time fee and yes still requires a bit of work learning how to use it. It's not that it's hard to use, it's just different and does so many different things that it can be a little overwhelming at first. However there is a real nice forum to handle support and questions as well as resources and tutorials.
Personally next to Trainz the price I paid for TransDem was the best investment I've ever made...