When you hold R1 to lock the camera, it seems to stop you using R2/L2 to move the camera up and down. I'm not entirely sure why that's necessary and you couldn't just not press those buttons! Maybe it locks the camera's altitude, so that if your train descends a hill, the camera stays at the original height?
I thought camera lock 'saves' your camera position. So if you set your camera on 'camera lock', go back into the cab and then hit the camera button, it should 'save' your previous view. I think that was the intention.
Camera lock prevents your camera from gaining or losing height when you move its position in space (rather than just looking around) to enable you to move only horizontally even if the camera is pointing up or down. Without it, if you are looking towards the ground slightly, as you move the camera forwards it will get closer to the ground. With it pressed and held, the camera maintains its height as you move forwards (or any other direction).