I think it would be a mistake for Disney not to have Runaway Railway ready for Summer of 2019 (so much so that I just assumed that was the plan to begin with).
Also, based on what we know about the ride, I struggle to see how it could still need another 16 - 18 months to be ready. The building's there and the sets inside are going to mostly be unthemed projection surfaces (which will move to some degree, yes, but still).