Is one sufficient per power line?
One per power district minimum. Ideally, you would want the TVS diode located as close to the device you are protecting as possible, or optionally between the source of the transients and the device... which can be difficult as the devices roll around on the layout. When I use them, I put them trackside, like at a lockon, or connected to switches(track terminals) where derailments are more likely or have happened.
