Trainstore works by moving the eng/wag files into a set of storage folders leaving only the ones you need to run your selected activity. Thus improving your loading time, as MSTS will load all of them. There is a comprehensive manual supplied with it take the time to read the installation section and you will have no problems.
To swap locos is a different matter - if the loco is picked up in an activity or is already part of the consist then you can switch cabs with the help of the BIN patch (see below) however if you want to drive a different Loco you would have to make a new consist - This can be done in activity editor - or download Conbuilder Lite [
http://www.conbuilder.3dtrains.com/] which is simpler to use. {when I was new I would make a backup of the consist I was going to alter - change the loco or stock and run it with the original consist name }
MSTS uses memory on your PC badly, goto
http://mstsbin.uktrainsim.com/eng/eng.html and download and install the BIN patch - this will improve the speed and framerates you get, as your new only d/load the latest stable version.
One important thing is to make sure you unstore everything if you alter your installation and always reinstall the latest T Sectiondat file after installing a new route - as this is totally backwards compatible with all routes - older routes will overwrite it and render any of the newer routes you install useless.
Hope this gives you a few pointers and doesn't scare you, Beginners guides and tutorials are here
http://www.webdottrainsim.com/ and here
http://steam4me.railpage.org.au/trainsim/index.html
