[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.

It makes sense why you see static bumping posts where only low-speed overruns are expected. They’re cheap; they’re simple; they don’t take much maintenance. But obviously, this is not a great solution for larger trains, and especially not for situations where speeds are higher. If you think back to our equation, where is all that energy going? Obviously, not all of it is leaving the train, at least in my demo. You can see it being bounced back a bit. The rest of it goes into heat and sound generated by the impact, vibrations into the table and track, and some plastic deformation of the post itself. It’s a bit hard to see, but it did bend over a bit in the high-speed tests. There’s really no energy dissipation built into the device. It just resists until something physically breaks.

There are some situations where that might be okay. I kind of dove into this without really taking the time to discuss what safety really means. That’s because there is no one answer in the case of a terminal track. The consequences of an overrun can vary a lot, and so can the consequences of a violent stop. On an industry track or yard, an end-of-track device may just be there to save the inconvenience of having to lift a railcar back onto the tracks. Nothing's moving very fast, so a rigid stop isn’t likely to damage the rolling stock. In passenger terminals, or places where buildings or public spaces lie beyond the end of the track, that calculus changes. You may have people on the train, and you might have people in its path. The design choices between stopping a train in an instant and limiting the violence of that stop are entirely a case-by-case decision. Even given a set amount of energy to dissipate, the requirements for acceleration will vary. So, there’s another simple equation to consider.

The stopping distance of a vehicle from an initial velocity is inversely proportional to the acceleration (or in this case deceleration). They’re on a see-saw. If you want to limit accelerations, the force that an end-of-track device applies to a train has to happen not at a single point in space but over some amount of distance along the track. Sometimes that happens whether you want it to or not. “Telescoping” is the term used to describe when railcars crush into or over one another. Like I said, the kinetic energy has to go somewhere. For freight trains in a true emergency situation, some amount of equipment damage might be acceptable. I can simulate that using a soda can. You can see that the accelerations go down a lot when you’re willing to accept some deformation. You can probably also understand why that’s rarely an ideal configuration, though.

One of the simplest alternatives is just to let the bumping post slide. And I can show you exactly what that looks like. I’ve modified my little demonstration here by adding some springs along the bolts that clamp this bracket down. Let’s run it again to see what happens. You can see that now, instead of an instantaneous hard stop, there’s some stopping distance. Looking at the graph, the peak acceleration is a lot lower, so this is much less violent for the train. And, unlike the hard stop, it’s easy to see where the energy of the train is being dissipated. It’s in the friction between the post and the track.

This is a pretty common arrangement in the real world, where limiting deceleration matters to protect passengers and equipment. The post or buffer stop is equipped with brake shoes that slide along the rail, burning off that energy through friction. Theoretically, this provides a constant level of deceleration that is independent of the train's mass or velocity. The friction is only a function of the clamping force and the properties of the brakes and rails. In the real world, though, that’s tricky to get right. You can see in my demo that there’s a spike in acceleration when the cart first hits the stop. That’s because it has to overcome the static friction first, before the lower kinetic friction takes over. You can also see the deceleration is a little more erratic. That’s the so-called stick-slip action of metal-on-metal sliding. You also have environmental factors like rust or moisture on the rail that can change the force curve. And of course, once something like this gets used, it has to be reset or, depending on the intensity of the collision, replaced, which can be a big job. There is another solution that addresses these challenges.

I’ve replaced the end-of-track device on my demo again, this time using a dashpot snubber. This cylinder that forces air though an adjustable hole to provide a nice smooth force curve. Let’s try it at a slow speed first. I’ve adjusted the stiffness to work well for this situation. Now let’s try a faster run. It looks a lot smoother. And you can see in the accelerometer data that it is. Here’s the rigid stop, the sliding friction run, and the dashpot run all together. The sliding friction had the lowest acceleration, which makes sense because it used up the longest stopping distance. But the damper provides a more controlled and consistent stop, especially when considering the range of train speeds and sizes. And it resets itself after the impact.

That’s why devices like this are most often used for passenger railways where control matters most. Usually, they are hydraulic cylinders that use oil to provide smooth deceleration across a range of train masses and speeds. Of course, using a hydraulic buffer stop doesn’t solve the distance problem. A device like this doesn’t really buy you more space. And in fact, if you’re trying to stop a high-speed train without extreme accelerations, you often need a lot more stopping distance than what’s possible with hydraulic buffers. In that case, sliding friction stops are really the best solution. You can see in my demo that it’s possible to bottom out the dashpot, and after that, it just becomes a much more expensive static bumping post. Hydraulics also add another layer of complexity and maintenance, so these are generally more expensive than sliding friction stops. And actually, you can combine the two into a hybrid system that uses the hydraulics for slower speed impacts… if you remember to tighten the clamps. Let’s try that again. This setup is kind of the best of both worlds. You get the control of the dashpot, while still allowing for sliding friction to dissipate extra energy in extreme situations.

There is another end of track safety solution at the other end of the complexity and maintenance spectrum: the earth mound, essentially a big pile of dirt. Never underestimate the power of a pile of dirt. If there is a large drop-off, important building, or some other critical situation beyond the end of the track, sometimes you don’t want to rely on a modest bumping post or buffer to make absolutely sure a train stops. A simple pile of earth doesn’t do much to protect the train, but it will definitely protect whatever’s behind the pile of earth, and sometimes that’s the thing that matters most.

Of course, there’s a lot more to this than the demos I’ve shown. Unlike the automatic "knuckle" couplers common in North America, many train cars are linked using chains. Managing slack in the train requires shock absorption on the cars themselves, and these devices are also known as buffers. There are also situations where you need to prevent a train from reaching a certain section of track that isn’t at the end of a line. A derailer is a device that forces train wheels off the track using a wedge-shaped metal block. Again, it’s likely to cause damage to the rolling stock, but there are situations where safety is worth that cost. A controlled derailment at low speed is far better than a runaway car endangering workers, crossing a busy road, or colliding with a high-speed passenger train on the main line.

There are also non-structural solutions to train overruns. If buffers and derailers are the "brawn," Positive Train Control is the "brains.” These systems use digital logic to take over train operations if a human operator makes a mistake or is incapacitated. PTC relies on GPS, trackside sensors, and onboard computers to "know" exactly where the train is, how fast it’s going, and what the conditions are ahead. If the system detects that the train is exceeding its authority, the PTC will automatically apply the brakes.

When you’re moving big heavy stuff around, and especially when that big heavy equipment has people inside or nearby, there is a ton of thought and engineering that goes into the safety systems, their redundancy, and the unavoidable physics tradeoffs between energy, acceleration, and space. Whether it’s the elegant, constant pressure of a hydraulic piston or the raw, grit-and-metal friction of a sliding stop, buffer stops and bumping posts are the last line of defense against the unimaginable energy in modern trains. Digital innovations like Positive Train Control are helping to ensure these devices never have to be used, but as long as we rely on rail (and I hope we rely on it more in the future), the "simple" physics of stopping will always be a complex balancing act.