If you find that you are unable to move the camera using the mouse, then you may have VR, eye tracking, or TrackIR software installed. Only TrackIR is officially supported by Train Simulator.
  1. Please try removing all peripheral devices from your computer except the monitor, mouse, and keyboard. 
  2. Next, uninstall any VR, eye tracking, or TrackIR software you have installed.
  3. Restart your computer and launch Train Simulator

Additionally, using a controller may affect your ability to use the mouse to control the camera. Therefore, please make sure that your game pad is disconnected when playing, unless you're choosing to play with a controller.