This video popped up in Youtube:

The tunnels the video refers to are subway tunnels (WTC Cortlandt station on line 1); to illustrate this, they used an image of a Class 89 locomotive in InterCity livery.The Collapse of World Trade Center | The Complete Physics
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13:32 they've strengthened the concrete walls
13:34 using tieback cables the engineers were
13:37 able to locate a strong bedrock at a
13:39 depth of seven Stories the concrete
13:41 foundation of this building progressed
13:43 from there the engineers called this New
13:45 Foundation Technology a bathtub
13:48 interestingly the bathtub Foundation
13:50 also accommodated two tunnels this
13:52 strong Foundation was the main reason
13:54 why the impact force of such a gigantic
13:56 airplane couldn't topple the building
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