ST1594 : Miners wheel
Great Western Railway please - not Rail Road! Brunel would be spinning in his grave! Looks like the second R is a bad case of artistic licence....
[Edit] on closer examination, the RR is in a different font and judging by
ST1594 : Wheel Drums, Hengoed Viaduct, South Wales
it is not connected with the GW. The RR almost certainly refers to the Rhymney Railway which also served this area.