Originally Posted by
pastorbobnlnh
...and to just clarify a bit more, the rails are steel with a copper wash or plating or coating of some sort. Not being a metallurgist, I'm uncertain of the correct term or process.
EDIT: Just checked the Brooks link posted by @
rekmeyata and the process is plating.
They would have to be some sort of plating (maybe electroplate?) because copper by itself would be too soft and too malleable to hold a riders weight or take the seatpost clamping from crushing it.