When you select a route in railsim, you have two options to play - the Free Play button and the Scenarios button. Free Play is merely a shortcut to the first Free Roam scenario in the list, if you click Scenarios and then select the top scenario in the list, that's the same as clicking the Free Play button. If you go in thru the Scenarios list you can actually have as many Free Roam scenarios as you want, starting in different locations with different weather, season, time of day, and services. To do this you need to learn to make your own Free Roam scenarios.
Start anywhere, preferably someplace close to where you want to start your scenario. Here I've started in the default Free Roam scenario, move the cursor to the bottom and click the globe, or hit CTRL E to enter the world editor (route editor).
At first the route will be locked so you can't edit track or scenarios, to unlock it click the padlock icon in the lower right corner.
After unlocking, move the cursor to the upper left to pull out that flyout and click the Scenario Tools icon. You know you're in the Scenario Editor (activity editor) instead of the world editor when you see the VCR player. Not going to go into what that's for, don't need it for a Free Roam scenario, it's just a handy way to know which editing mode you're currently in.
To place engines and cars that are not default to the route you're working in you need to select the Object Set Filter from the left middle flyout, then go to the right flyout to select the subfolders the trainsets you want to use are in. You only need to do this once for each route unless you add new trainsets with a different developer subfolder afterwards.
Holding down the right mouse button to steer I use the up arrow key to fly to the location I want my new Free Roam scenario to start, SHIFT up arrow to fly faster. Arriving at Barstow, I want to hide the area marker and sound region baskets, so I click the Display Filter and uncheck the boxes for the visual clutter I don't want to see.
In the middle left flyout select the scenario marker, Free Roam, and place it on the ground. The game goes into a Twilight Zone effect for a moment because you are now leaving the scenario you were in and entering the new one. RIGHT click to cancel the impending action, because as soon as you place one it readies another! After that, reselect the marker and raise it up and/or rotate it, double click on it and type name, description, and briefing. Also edit weather, season, time of day if desired. Hit F2 to save.
Now what we're here for, it's all about the TRAINS! I downloaded some ATSF repaints and I want to drive those, so select one from the engine list and place on the tracks. Again, RIGHT click after placing if you don't want to place more than one. Reselect and doubleclick an engine to get orange arrows above it, click the orange arrows to reverse one. Select the driver icon and attach to the engine to make it a driveable service in the game.
Switch to the cars and place cars behind the engine, sliding them up until you hear the coupling thump. If you have coupling override ON in the options you can couple different coupler types together in the game, but not in the Scenario Editor, so if you want to mix European cars in you'll need to couple up to those manually after starting the game. When you're done, F2 to save.
Instead of clicking Free Play, click the Scenarios button, find your new Free Roam scenario in the list.
After playing with it for a while, make another one, practice makes it MUCH easier.

From everything I read this is not possible. What I would really want to do is set up a regular service back and forth passing through my main yard and still have the ability to 'play trains' in the remainder of the system. The regular service doesn't stop, it simply passes through the yard once in each direction once a day, also the tracks used by the regular service do not interact in any way with any of the remainder of the network...
How about if I setup a standard scenario with the scheduled service included and a designated drivers engine some rolling stock and some kind of dummy task which I ignore?
Never done anything with scenarios so any pointers are welcome...
Geoff