The Glenfinnan Viaduct was not reinforced concrete, because arches if designed properly only carry compressive loads, although the headframe structure must be reinforced, due to its length to cross section ratio, even if it is always in compression.
I have not been able to establish when reinforced concrete began to be used in the UK for applications like that, but am inclined to think that it may not be all that old. About 1920 would be my guess.