The MSTS Activity Editor allows you to make your own activities, or missions, for the player to complete. This is also how you can get lots of other trains running while you drive yours!
The Activity Editor is generally quite easy to use, you can probably work it out by fiddiling around with it for a hour or so that how I did it. Uploading is fairly easy too just make sure you are logged into UKTrainsim click on Upload File on the front page and fill in the nessecary information. It won't be avaliable straight away as it has to be approved by a moderator first.
Copyright isn't usually an issue with activites as you are not using any files that are covered by copyright, you are not distrbuting the route or locomotives with the activity, only the files required for them to appear in the users simulator ie they must have the relevant route and stock before they can run the activity.
Hope this helps and good luck with your activity creation.
There is also some information in the beginners guide which you can access from the main home page. I learned by opening an existing activity and, using the inbuilt help, checked each step against the actual activity which i found made it clearer.
Then start with a small activity on an easy route - with double track, as single track activities are much harder to get working right. As your experience improves you can move on to better stuff.
