A couple more details
In sessions (what MSTS calls "activities"), there is nothing like the driver commands in Trainz to make an AI train go here, pick this up, go there, unload that, and so on. All you can do is set a path and the time that the AI train will start down the path.
That also means no triggers, so if your intent is to have the player encounter an AI train at a certain time and place, it had better be really early in the activity or the odds are the timing will get hosed up and the AI train won't be where you hoped it would be when the player arrives.
In MSTS, AI trains cannot couple to cars that weren't part of their original consist. So even though you could theoretically create a path for them that would result in a coupling, the coupling won't happen.
There are also no industries of the sort we have in Trainz. In MSTS you can drive up to a coal mine, but don't expect any sort of loading or unloading animation. There's no such thing.
In fact there are no scripted assets of any kind, which means no auto-numbering of rolling stock, which means all your Santa Fe SD-40s will have the same running number, or you'll be reskinning numbers a lot.
If you think missing KUIDs are a pain in Trainz, just wait until you download an "activity" that crashes MSTS altogether without telling you why. If by some miracle you find out it's because you're missing some piece of rolling stock, you can probably find it on trainsim.com. But if it isn't there, good luck.
And be thankful that Content Manager does manage to find most of the KUIDs you need. MSTS has no Content Manager, so if you download a route or an activity that somehow got packaged up without all its dependencies, you have to manually hunt down each one and install it by hand.
I lived with all those shortcomings for years and still thought MSTS was good fun. But I wouldn't go back.
