Q: Engine Bob, I commute to New Jersey from Penn Station, which means my train runs beneath the Hudson River. Somebody once told me that those tunnels are so shallow that they’re not anchored to anything and they “swing” like garden hoses as trains run through them. It does sort of feel like the train is bouncing as it runs thorough. Don’t tell me this story is true.
A: Okay, I won’t—because it’s not. The bounce you’re feeling is, alas, only the bounce of the suspension assembly on the pair of wheel trucks of your train (Train cars have suspension just like automobiles do, because even though rails are obviously smoother than a paved road, your butt riding on a steel wheel that hits an inch-wide rail gap at 80 m.p.h. is going to feel it, so the wheel assemblies use springs or hydraulics to prevent you from getting wrinkles before your natural time.)
But, like I said, the answer’s no: Those tunnels—there are two of them, each about 23 feet in diameter—do not bounce. There is some minor “play” to them, but bounce is a bit extreme. (That said, it’s true that plenty of transportation structures have sway coefficients factored in; the Manhattan Bridge, for example, flexes a few inches as the subways run across it. Prior to its recent renovation, it was actually moving several feet, but that was, uh, a problem.)
Anyway, the true story of the purportedly bouncing tunnel is, however, almost as cool as the myth. Begun in 1903, those tunnels were the first real-world manifestations of an engineering patent granted to British engineer Charles Jacobs. His invention was called the “Tunnel Bridge,” actually.
Didn’t happen to notice a bridge when you were down there last? Well, here’s how it worked. Excavated for the Pennsylvania Railroad, the “North River Tunnels,” as they’d been known, were made of interlocking iron (and sometimes steel) rings that were bolted together behind a massive drilling shield, inside which workers hand-dug through the river silt that was kept from inundating them by a high-pressure atmosphere artificially created inside the tunnel by huge air compressors up on the ground. (When a worker failed to spend enough time in the pressure-equalizing airlock, he’d come down with “the bends,” which you might have heard of before.)
The tunnels rest at varying depths below the bottom of the river but, much like your last blind date, are surprisingly shallow—usually buried only about 20 feet in the muck. The bedrock is much further down, but tunneling through rock would have been impossible not so much because of the difficulty of the drilling, but because the trains couldn’t (and still can’t) climb grades much steeper than 2%, and diving down to bedrock level from either Manhattan or New Jersey would have required a really steep descents and ascents, since the river’s only about a mile wide.
Yeah, even in the Olden Days, life was just as complicated.
So that’s where the “bridge” part came in. Every 15 feet, workers fitted the iron rings of the advancing tunnel with a “bore segment.” This was a special ring through the bottom of which would pass a long, thick screw. The sandhogs would crank that screw straight down until it reached the bedrock, then they’d drill it in nice and tight (please, no second reference to your last blind date.) And that maneuver anchored the tunnels.
So, imagine your commuting tunnel now: It runs through the river mud, but since the mud’s not thick enough to support the enormous weight of the tunnel and its trains, you’ve got what are essentially legs—pilings, really, like you see on a pier—that hold the tunnel up and keep it in place. Sorry, then—no bounce.
But what’s really amazing is that those tunnels have been down there for a century. Hundreds of trains a day run through. And still—all safe.
I don’t know where Charles Jacobs is buried, but I’d leave him a nice bouquet if I knew.