Thankfully, the wall that separated the Hudson River from the subway did not fail. If that had happen the river would have flooded the subways tunnels.
Honestly, that slightly under 3,000 lost their lives in these attacks, ( Washington, PA, and NY) was somewhat miraculous.
Which was the objective of the July 07, 2006 plot to bomb the PATH train tunnel:
The men believed that by bombing the train tunnels, they could unleash a severe flood in lower Manhattan, including Wall Street, said the official, who spoke on condition of anonymity because the investigation is ongoing. The official also said investigators believe that an attack on a train tunnel, unlike the Holland Tunnel, could have achieved that goal.If they had cracked the PATH tunnel walls under the river, and flooded the tunnel, the whole lower-Manhatten subway system sits below the water line