Welcome Gary, to the forums and to Trainz.
With Trainz you can take control of any AI driven train on your route. You can also, which I'm not sure you discovered, climb into the cab with any of these drivers and ride along with them as they go about their plotted route.
To gain control of an active AI train, click on it from an external view, or click on the driver image on the left hand lower part of the screen, which will open up and you can scroll through the various drivers, if there is more than one, one the route. By clicking on a driver, this will put you in view of the driver.
Once you have a driver in your camera view, press the 1 key (Not the number pad 1), to put yourself in the cab.
The other camera views available are 2, 3, and 4 also across the top of the keyboard. Camera views 2 and 3 giving chase a camera and a tracking camera. Out of these camera views, outside of internal camera view 1, is camera 4, which is the free camera where you can roam around a route and travel anywhere while still attached to a selected consist. This means you can exit the cab, go to a siding and flip the switch, then press 1 again to back into the cab and continue driving yourself, by choosing stop and then taking over the control of the train.
Now these are only the basic operations. There are so many, many things you can do with this program that you'll spend many hours and years (hopefully) using it. By years I mean a decade or more. Yes, many of us have been here since the early days of Trainz. I started in December 2003, while others have been here since the program's inception in 2001.
