The bridge in the next post may look like an ordinary bridge, but it’s quite important to travelers in the Northeast. All Amtrak trains going south from New York and north to New York cross this bridge, as well as 2 high volume lines of New Jersey Transit. Every month or so, a truck driver will over-estimate the clearance and slam into the bridge, resulting in a “can opener”. When this happens, the authorities have to come in and do an assessment of the bridge to ascertain that it’s still structurally sound. Until this assessment is completed, trains still pass over the bridge, but at extremely slow speeds ~5 mph. Why? Intuitively, wouldn’t it make more sense for trains to speed over the bridge, minimizing the amount of time that the bridge has the weight of a train on it?
By the way, there is no clearance height shown on the bridge because truck drivers from both directions are told to turn and cross at another location. Some of them just miss these signs.
A train moving slowly adds to the downward force on the bridge by a constant amount.
But a train moving rapidly may set up resonance frequencies that cause the load to fluctuate. The average downward force over time is the same as a slow train, but you could have momentary spikes of increased force that could cause a critical component to fail.
Going slowly allows the engineer ample time to visually inspect the track ahead for buckling or gaps. It also significantly reduces the chance of a catastrophic derailment.
This all assumes that the bridge structure itself is not likely to collapse. Bridges are designed with some amount of crash impacts in mind, so the old can-opener trick shouldn’t cause the structure to fail. But the jolt can definitely cause the tracks to get screwed up.
My railroad analyst here at the office tells me friedo is correct, although Pochacco also makes a good point.
Can’t they put up a gauge? The el I used to take to go to school in Philly came out of the subway in front of 30th St. station (until they extended the subway around 1960 and it came out a mile further away). Where it came out there was a very low section and a small distance away there was a gauge erected so that if you hit the gauge (which consisted of some suspended boards painted yellow) you were not going to pass under the subway. Every couple weeks some truck detroyed the gauge, but it was a lot cheaper to replace it than to replace the el.
Sound like good answers. Thanks.
For some bridges, “warning” barriers are placed about 50 yards upstream, so oversized truck will hit these and be warned to stop before slamming into the bridge. Since the cost of this looks to be less than even a single bridge damage/repair cycle, I’ve often wondered why they aren’t more common.
Having grown up very near the overpass in question, way cool to see it here!
I’m surprised that you say it’s as often as once a month. I certainly remember it happening more than once, but I wouldn’t have said more than once every couple of years. Of course, I haven’t lived there full-time in over a decade. I’d think the installation a few years ago of the traffic light that’s right before the bridge, by the grocery store, would have decreased the can opener incidents even more, because you have plenty of time to take a good look at it. (It’s a stupid light that takes far too long, so I hope it’s serving some purpose!)
You also have to consider what would happen if the bridge did collapse. If you are going very slowly, the damage isn’t that likely to be very severe. However, if you were booking it across the bridge, you might get a serious derailment when the engine went off the tracks.
This. A derailment at 5mph is unlikely to injure anyone, whereas 50mph would be disastrous.
It looks like a sturdy bridge, too – with 12 support beams, the displacement of one is unlikely to trigger a catastrophic collapse a la I-35. (Presumably, if any beam was visibly shifted out of place, they would not allow trains to cross the bridge at all.) It’s also very short – only the length of a single railroad car, which minimizes load on the bridge itself.
Am I the only one wondering why they don’t put signs directly on the bridge saying no trucks are allowed under, just in case those truck drivers missed the initial sign and don’t try to pass under the bridge?
Thinking about it some more, I experience a commuting delay resulting from a truck crashing into the bridge about once a quarter. But it probably happens more often than that since they’re not all going to coincide with my commute.
Before I got married, I used to live in one of the apartment buildings within walking distance. I still park at that station because of the incredibly cheap parking $40/month! Of course, you have to factor in the costs of your car getting stolen from time to time.
Wow, thank you so much for posting this link, I immediately recognized the bridge you’re talking about, having crossed under it nearly every day on my way to high school.
Small hijack: Is this the underpass that floods every time it rains, yet the truck drivers still insist on attempting to drive through, or am I thinking of the underpass on McClennan Street?
I think that’s the one on McClellan Street, although there are puddles that accumulate around this one too.
Both underpasses are notorious for flooding, although it seems that they’ve finally fixed that problem on North Ave.