3D Train Studio is a powerful tool for designing, constructing and simulating a complex 3D model railway system, including moving trains, working signals, sound effects, even cockpit cameras to help you monitor what's happening in your custom world.
Putting all this together and making it run smoothly can take a very long time, but fortunately the program provides a few shortcuts. In particular, when you're creating a new project, 3D Train Studio provides a "Recent Projects" section which lists some largely finished railways created by others. Double-click one of these and it's opened in a few seconds.
Hold down the right mouse button and drag (or use the cursor keys) to move the camera around, spin the mouse wheel to move it backwards or forwards. You can also select any objects - trains, track sections, buildings, signals or scenery - to move or manipulate them.
Alternatively, just click View > Cameras > Cockpit Cameras and choose one of the trains. (There are also vehicles here which aren't trains; if in doubt, just keep choosing entries on the list until you find something that's on a track.)
Once you're happy, set the "Speed" value bottom right, the train will begin to move, and you can enjoy your scenery from the cockpit. Or switch to an external view to see the train in relation to other objects. We found this wasn't always very interesting - some trains only make very short trips backwards and forwards on the same line - but choose another cockpit and you'll quickly discover something better.