Placed alongside a minor bridge carrying the old LNWR 'Heads of the Valleys' railway over a private lane. You do not need to go down to the road level - the cache is to be found above the road, but below the railway. Happy hunting.
For a short history of the Merthyr, Tredegar and Abergavenny Railway (Heads of the Valleys Line) see - https://en.wikipedia.org/wiki/Merthyr,_Tredegar_and_Abergavenny_Railway