I don't understand his use of a tripod. Well maybe he is shooting video.

Good point. Sometimes on rare occasion I find a rail composition with distracting elements on the periphery (like parked cars with shiny chrome, high tension power lines, a billboard, etc.) and if I have a tripod, I can perfectly set the scene and trigger the shutter as the train passes through. Without the tripod, I can't hold everything as I want. But, normally I shoot handheld.