We’re asking experts some of the toughest questions we’ve heard about the Gateway Program. Here’s the first in a series of q&a on Gateway. For a refresher on what Gateway is and why it matters take a gander over here.

Question: Why can’t we just build one tube instead of two?

Answer: It would be unsafe and limit future capacity.

Modern safety codes (NFPA 130) require that tunnels have an emergency exit every 2,500 feet or cross-passageways every 800 feet. This is a critical safety requirement for the new Hudson River Tunnels that will span a distance of over 5,000 feet beneath the river. If only one tube was constructed the agency would be required to build an exit stairwell with a refuge in the middle of the Hudson River. This would be very expensive due to the depth of the tunnels (~ 90ft) and need to protect the shaft from collisions with ships or floating debris. It could also act as a chimney during a fire unless additional actions were taken to ventilate the smoke away from the exit shaft. Cost and constructability concerns aside, an emergency exit in the river would ultimately be infeasible because it would obstruct the navigation channel of a critical waterway – the reason why we have mostly built vehicular and rail tunnels to traverse the Hudson in the first place[1].

Engineers are well aware of this issue (it impacts many other projects worldwide) and have devised an alternative that uses cross-passageways, or small connecting tunnels with fire doors, to bridge the two tubes and allow for the riders from one tunnel to evacuate to the adjacent one in case of emergencies. The new Hudson River tunnels will have 15 cross-passageways that will be spaced at intervals of 750 feet. Two tubes will ensure that in case of emergencies customers will be able to cross into the other tube and board an evacuation train. This design and evacuation procedure has been proven in facilities around the globe such as the English Channel Tunnel or Chunnel (that also has a smaller third tube that is used as a service tunnel) and right here at home on the recently opened Second Avenue Subway.

Building one tube would also reduce the redundancy that having multiple tubes brings. While the existing North River is under construction, if the new tunnel was damaged due to a large-scale weather emergency or other unexpected event, passengers would be subject to delays or worse, being completely cutoff with no back-up.

One tube will also severely limit the capacity of Gateway once the expansion of Penn Station New York (PSNY) is completed. The new tunnels will enter PSNY south of the existing interlocking that limits their access to the station’s northern tracks. They will be aligned to serve the central and southern platforms and the recommended southern expansion of the station. Without the additional tube, the capacity of the station will be decreased substantially, potentially by more than half of what’s currently planned. This will result in less reliable service and more crowding along with likely eliminating most if not all of the planned one-seat ride service improvements for customers on the Bergen-Main, Pascack Valley and Raritan Valley Lines.

Question: Why not connect the new single tunnel to the old ones?

Answer: The reason here is straightforward, the new tunnel alignment is not adjacent to the old ones for many reasons, and safety is one. The North River tunnels are over 100 years old and there are serious concerns about disturbing the earth around them. Any actions that would require breaching the structure of these old tubes is considered to be too risky since it could result in their failure.