The Camera Menu allows you to change between the different camera modes as below.
Places you into the Interior/Cab view of either the leading vehicle of the train, or the selected vehicle if it is fitted with an Interior view.
Places you into the Chase Camera mode. This camera is fixed to the selected vehicle, and allows you to rotate around the vehicle and zoom in and out. You can mode between vehicles in the consist using the "-" and "=" keys or by left clicking on the vehicle you wish to go to.
When available, this camera mode will use trackside (static, or tracking) cameras to view your train go by. If no trackside camera is nearby, then it will revert back to Chase camera until the train gets close enough to a trackside camera.
Roaming camera allows you to move freely around the route.
The Map view allows you to view a map of the route. This view can allow you to set junctions and view signals ahead of you, as well as view the location of industries, other trains, and more.