I believe that all the tanks along the original WW&F were steel with a wood shelter. Reportedly, the base of one tank still exists in Weeks Mills, ME - used as a garage. My understanding is that most if not all of the steel tanks were removed during the WWII scrap drives - leaving the buildings to collapse (as happened at Head Tide - which our tank at Sheepscot is a mirrored-replica of.)
As shown in Brendan's photo, the tank in our replica is new. We initially intended to use a older tank of unknown (non-RR) origin that we acquired from the SR+RL RR museum. However, it was deemed to far-gone to use - so we purchased a new steel tank - and we now use the one we acquired as an impromptu shed.