I bought MSTS the day it came out, and while I did like it, it didn't stay on my HDD all that long. Just "something" missing for me, and I think that "something" was the add-ons. I am not very computer savvy when it comes to software, and, like, MSFS, if an add-on wasn't an "exe.", it was usually beyond me.
Not sure, from the demo, if Trainz is any more user friendly, but I do like what I saw (the Big Boy session).
Here's one, not sure if this is where I got it, or not. Google, and take your choice.
There is a HUGE number of add-ons available from MSTS at train-sim.com in their file library. There are also many payware add-ons for MSTS from places such as
3dtrains.com
3dtrainstuff.com
mapleleaftracks.com
trainartisan.com
streamlines.ca
vscalecreations.com
dieselswest.com
And many, many, many more payware and freeware companies. Do a Google search and you should find plenty of them. The quality compared to the original MSTS equipment/routes is unbelievable. Check out some screenshots at railroadforums.com/forum in the screenshots section.
Also, don't forget to download MSTSBin which adds new features to MSTS.
It is at http://mstsbin.uktrainsim.com/eng/eng.thml
Finally, I will be making some freeware (and later payware) add-ons, my website, Wisconsin Locomotive Works, is geocities.com/trainsim1. WLW's forum is at http://s7.invisionfree.com/trainsim1_MSTS_forum.