Rolling stock varies considerably regarding internal cameras. Powered rolling stock usually has at least one internal camera in the cab. Passenger carriages sometimes have internal cameras, sometimes not. Quite understandably goods wagons hardly ever have internal cameras. It's up to the creator how they configure the cameras and the user cannot change that (well, not without content creation skills).
First, in external camera mode, click on the relevant vehicle in your train. Alternatively press the + or - keys on the main keyboard to move the camera up and down the train.
Then switch to internal camera mode. If the vehicle hasn't got an internal camera, nothing will happen.
Then press the [ or ] keys to rotate round the set of internal cameras in that vehicle. If there is only one internal camera, nothing will happen. Confusingly, sometimes an "internal" camera is physically located outside the vehicle.
Then rotate and tilt the camera angle with the cursor keys as normal.
Hope you didn't already know all that! - John