I've never used a multiplayer session so I can't say yes or no, but a basic driving session setup to have AI run from one portal to another would do that.
What I do is setup something as just that. I have my AI driving from one end of the route to another on the mainline while I take control of local and yard shunting. There are AI passenger trains on the same line I am, for instance, which I've setup to stop at various stations, and AI controlled freight consists running from point a to point b, or point b to point c as they follow the various driver commands. Staying out of the way of the AI traffic, can become a challenge in its self, and has become part of many of my driving sessions.
For complex routes, I highly recommend the Driver Schedule Library and the related Copy Commands driver command. Using the library, you can easily setup a bunch of 'schedules', which are then saved and loaded into the driver command bar via a single copy commands driver command. This saves a ton of time when configuring multiple drivers, and portals.
So with all the AI drivers running, I'll as I said take over the local switching as the AI pass by the yard. With the wonderful ability to take over AI trains at any time with Trainz, I'll stop a through running AI train, uncouple the engines and direct the AI to temporarily drive to the engine house while I then switch out the freight wagons. When I'm done doing my job here, I'll then have the AI driver couple on to the freight consist, and continue on the schedule.
As I said, this is done in a basic driving session simply setup with commands and schedules. There is no end-goal of driving a train at a set schedule, no golden coins to collect, sorry I mean points and stars, and no little floating guide points lofting in space, which get hidden by the driver image and screen because the interface changes, no text that's so small on these things that it's about the size of ants' eyes.
My sessions can last 8 hours or longer on my 200-plus mile route. I may not see the complete route, however, the AI are busy driving on the other parts as I focus on one area. On one route, for instance, I have numerous coal mines on various branches located in what are called hollows. I'll run the coal trains from the various mines while the AI are busy tying up the mainline on this tiny section. If I get tired of moving coal trains, I'll move on to a through passenger train, which I'll 'catch' as the AI passes by. I'll take that ride to another part of the route where I'll take over switching and driving there.
The thing to keep in mind is sessions like this take time to setup. It's not that it's complex, with hundreds of triggers and special effects, it's just a lot of commands. When setting up a long driving session, or one on a larger-sized route, setup a couple of portals, and one of the driver schedules. Test this with multiple consists, then once this is successful, go on and add more drivers, more schedules, and so on to build up the driver session to its fullest.
The other thing is never use the default session for your driving. Instead copy the default session to an active driving session with another name. Why you ask? The reason is simple. When working on a route, the default session can and will be overwritten, sometimes on purpose initially, but usually by accident. There's a lot going on here, and in the haste of saving your hard work, this is one less thing to think about and you don't want to lose your hard work that you spent days on, and found a simple route thing that needed fixing like a bridge abutment facing the wrong direction, and saved and forgot to pick do not save session.
) unless you shorten the list and add different platforms at the end.