Actually it is not that bad. It's not exactly like running a steam locomotive since I believe it was designed more for diesels and electrics and I find that the trains stall too easily.
Controls are keyboard mapped and after running a couple hundred miles, you can control your train without needed to hunt for keys. Or you can drop a few hundred dollars for the cab interface (although it's more of a modern diesel cab.)
To get the "expansion packs" from train-sim.com, you need to join which costs like $20 for the year and it gives you access to stuff people have developed and put out there for others to use. Everything from everywhere! You'll need to get the x-tracks and newroads files for sure for the narrow gauge railroads. For the actual locomotives, rolling stock or other files, when you begin to download, read the "Readme" files and they'll explain what you need and what to do to install them. Some of them need the Train Artisan New York Central train pack which I had to buy from Just Trains in England. From what I understand, there are more folks from Europe into the train simulations, probably because more folks actually ride their trains there!
I have found that I prefered to use the K-36 cabs on some of the C or T class locos which use the default Orient Express loco cab (European.) You can switch out cabs and tweak the whistles and so on by getting into the .CAB files. I got my #20 set up pretty nice now using the Eureka's whistle and so on. Of course, I tend to run the K-27s with freight on the RGS. I also changed the number of notches on the quadrants to make it easier to adjust faster (rather than the 1% at a time...) (Play with the settings but be sure to have a backup copy stored somewhere else.)
There is also a MSTS group on Yahoo! which discuss how-tos and so on.
From what I have read, MSTS is still the best for all of this. When you can't be there with the real thing, this is a pretty good way to keep the required trains in the blood!
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If you have any questions, email me.