Factual error: In the scene under Trinity Church, Ben is holding on to the elevator and trying to get a grip to swing to safety (after Dr. Chase is already to safety). He grabs a board that falls off, then the last board pulls back an is shown to be held on with modern day nails. If it were built 200 years before, as the movie suggests, these would not likely be the kind of nails from the local hardware store as shown in the film.
Factual error: When Riley is sitting outside the Franklin Institute and sees the ad on the side of the bus with the close up of the Liberty Bell, the SEPTA bus is labeled with the 108 Airport route. That bus route goes from the 69th Street Transportation Center to the Philadelphia International Airport and would not pass near the Franklin Institute.
Factual error: The poem on the pipe stem reads Fifty Five in Iron Pen, and Nicolas Cage states "a document that fifty five men signed." The Declaration of Independence was signed by fifty six men.
Answer: He doesn't simply assume "Beneath Parkington Lane" means beneath the church: Parkington Lane is the name etched on the tomb hiding the entrance to the tunnels. When he saw it, he naturally deduced what he had to do.
Sereenie