Within a scenario, is it possible using the Build Menu, to change the configuration of a fixed Junction (non moveable) from inactive to active? In other words, can it be made to change tracks?
Automatic switches (the ones without a lever) are controlled by the game dispatcher, the only time you can manually change them is in free roam scenarios as far as I'm aware.
If you are making or editing a scenario a train can normally be routed either way thru points. As knuckleshed said, "the only time you can manually change them is in free roam scenarios" In a standard scenario the route is already set in the timetable editor and cannot be changed when playing. Unless you are meaning something else?
If the junction has a point motor and is switched against the pathing, clicking on the motor can sometimes switch the junction to the correct pathing in a Standard scenario. If the junction is locked by another service you'd get a wee window appear with the details of which service was taking precedence.