GoKawiil[Note that this article is a transcript of the video embedded above.]
On the morning of September 29, 2016, a New Jersey Transit commuter train rolled into Hoboken Terminal during rush hour with about 250 passengers on board, and it just… didn’t stop. The train reached the end of the track, overrode the bumping post, and slammed into the terminal wall at about 21 miles per hour. Debris fell onto the platform, killing one person, and 110 passengers and crew were injured.
Just three months later, essentially the same scenario played out again, this time at Atlantic Terminal in Brooklyn. A Long Island Railroad train crashed into the end of a terminal track. The lead car actually rode up and came to rest on top of the concrete platform. 108 people were injured, but thankfully, there were no fatalities.
In both investigations, the NTSB reached a simple conclusion: the trains didn’t stop because the engineers fell asleep. They were diagnosed with sleep apnea after the crashes, a chronic disease that leads to sleepiness and fatigue. But the NTSB also pointed to the absence of safety devices or systems that could have intervened to stop each train before the collision.
When you picture a railroad, it probably feels endless. Rails stretch for miles in both directions, with plenty of room for mistakes to unfold. But every line has an end. No matter how good the brakes are and no matter how experienced and well-trained the crew is, any system that depends on an individual, biological, fallible human, is eventually going to fail.
That’s why end-of-track protection is such a unique engineering problem. You might say that those bumping posts at Hoboken and Atlantic Terminal failed, but in reality, they were never meant to “catch” a fully powered passenger train at speed. They were a last-resort backstop, designed for low-speed situations. Train brakes have a lot of redundancy, and failures are rare, but those brakes only work if they are applied in some way. If it doesn’t happen, a static bumping post doesn’t offer much protection, and may even make things worse. End-of-track protection seems like a simple problem, but the solutions are pretty complex. And of course, I built a little model in the garage so we can take a closer look at them. I’m Grady, and this is Practical Engineering.
In many ways, stopping a train is a pretty elementary physics problem. You have the train’s mass, its speed, and an elementary equation: one half mass times velocity squared. That gives you the system's kinetic energy. And in order to bring a train to a stop, all that energy has to leave the train and go somewhere. But this can vary by orders of magnitude. A single empty freight car inching along in a yard may only reach kinetic energies on the order of a hundred kilojoules. A tram or streetcar experiencing a low-speed overrun, like a partial brake failure, may take a few hundred kilojoules to stop. For light rail, you’re around half a megajoule to stop one moving at terminal approach speed. Heavier commuter rail trains may be 5 to 10 times that. And heavy-haul freight trains can climb into the hundreds of megajoules even at relatively low speeds.
You can see why the design of end-of-track devices, often known as bumping posts or buffer stops, is deceptively complicated. The ability to bring a train to a halt in an overrun scenario depends on both the mass and speed of the train. Ideally, if a train is approaching the end of the line, it’s not moving at track speed, and that is the assumption that most designs are based on. That’s why simple bumping posts are so common. They’re like a door stop for a train, just static devices that provide a hard stop at the end of the line. Let me show you an example:
This is my train stopping simulator, which is kind of just an excuse to play with a pneumatic piston. At one end, is that piston, and I can use a regulator to adjust the force this applies. For my “train,” I’m using a little cart on a length of V-rail with some weights.
Let’s set up a little bumping post and see what happens. No surprises here. At low speeds, the train stops nicely against the post. Let’s try it at a higher speed. The collision is a little more violent. And actually, we can quantify that. You may have noticed the accelerometer at the top of my train and the laptop with squiggly graphs in the background. Here’s the data from that smoothed out a bit from the raw readings. Again, at low speeds, it’s not a big deal. It takes about 5 gs to bring the train to a stop. At higher speeds, it was maxing out my accelerometer at 16 g.
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