The poster advertises the Explorer Boats in Adventureland. Based upon this bit of photographic evidence and the posting by Ken at Outside the Berm of the Disneyland Fire Department sandwich board sign, I believe this Frontierland sign with the Railroad engine was on Main Street. The sign reminds me of the Railroad cards seen in Monopoly. The sign measures approximately 22 by 14 inches and is hand painted on the grey card stock.
If anyone has documentation of this sign, I would love to see it!!