GoKawiil[Note that this article is a transcript of the video embedded above.]
If you’ve ever ridden a bike, driven a car, or operated pretty much any other vehicle on earth, there’s a fact you’ve probably taken for granted: you can see farther than it takes to stop. Within the span between seeing a stationary hazard and colliding with it, you have enough time to recognize it, apply the brakes, and come to a stop to avoid a collision. Your sight distance is greater than your stopping distance; it sounds almost silly, but this is a critical requirement for nearly all human-operated machines. But it’s not true for trains.
Engineers can see just as far as the rest of us, but the stopping distance of a fully laden freight train can be upwards of a mile. That means if an engineer can see something on the tracks ahead, it’s often already too late. So, trains need a lot of safety infrastructure to make up for that deficiency. For one: trains almost always have the right-of-way when they cross a road or highway at the same level, or at grade. The cars have to wait; And, we use a litany of warning devices at grade crossings to enforce that right-of-way and try to prevent collisions. In most cases, these devices have to detect the impending arrival of a train and give motorists enough time to clear the tracks or come to a stop. It sounds simple, but the engineering that makes that possible is, I think, really interesting, and of course, I built some demonstrations to help explain. This video is part of my series on railroads, so check out the rest after this if you want to learn more! I’m Grady, and this is Practical Engineering. Today, we’re exploring how grade crossings work.
It’s inevitable that roads cross railroad tracks, and it’s just not feasible to build a bridge in every case. In the US alone, there are over 200,000 grade crossings where cars and trains must share the same space. A car to a freight train is an aluminum can to a car: in other words, there’s a pretty big disparity in weight. So we’ve put a lot of thought into how to keep motorists, cyclists, and pedestrians safe from the trains that can’t swerve or stop for a hazard. You’ve probably stopped for a train at a crossing, but you may not have consciously added all the safety features up.
Of course, the locomotives at the front of trains themselves have warning devices, including bells, bright headlights, smaller flashing ditch lights, and most noticeably, the blaring horn. The standard pattern at a crossing is two long blasts, one short blast, and one final long blast. But the crossing has warnings too. Passive warning devices don’t change with an approaching train. They include a stop or yield signs, the crossbuck, which is the international symbol for a railroad crossing, and sometimes a plate saying how many tracks there are so you know whether to look for one train or many. Another crossbuck is usually included as a pavement marking to make sure you know what’s coming up. Many low-traffic crossings have only passive safety features, leaving it up to the driver to look out for trains and proceed when it’s safe. But, many crossings demand a little less margin for error. That’s when the active warning devices are installed.
A typical grade crossing features both visual and audible warning signals that a train is coming: red lights flash, a mechanical or electronic bell sounds, and usually a gate drops across oncoming lanes. That seems pretty simple, but there’s quite a bit of complexity in the task and the consequences if anything goes wrong are deadly. And the first part is just knowing if a train is coming.
Detecting a train is important for grade signals (it's also important for signaling trains about OTHER trains, but that's a topic for another video). It can be handled in a bunch of ways, but the simplest take advantage of the electrical conductivity of the steel rails and wheels themselves. A basic track circuit runs current up one rail, through a device called a relay I’ll explain in a minute, and back down the other rail. When a train comes along with its heavy steel wheels and axles, it creates a short circuit, a preferential path for the current in the track circuit. That deenergizes the relay, triggering all the connected warning devices or signals. But why use an ordinary old diagram when you have a model tank car, and an old railroad relay you got off eBay? Let me show you how this works in a real demonstration.
On the left, I’ve hooked up a power supply to the tracks, putting a voltage between the two rails. On the right side, I’ve attached a relay. Let’s take a look inside it to see what it does. I love playing with stuff like this. At its simplest, a relay is just an electromechanical switch: a way to turn something on or off with an electrical signal. When I energize the coil (at the bottom), it acts as an electromagnet, pulling a lever towards it. On the other side of the lever, you can see the movement interacting with several electrical contacts. It’s a little tough to see here, but these contacts are like switches that can control secondary circuits. Some will be switched on when the relay is energized, and others are switched off. When the relay is energized or de-energized, it basically flips the switch on these circuits, allowing various devices, like lights, bells, and gate arms, to be activated or deactivated. In my case, I have a simple battery and LED to indicate whether or not a train is being detected on the rails.
When there’s no train, current passes through the relay from one rail to the other, energizing the coil and holding the switch open so the LED stays dark. When I put a railcar on the tracks, the circuit changes. The wheels and axles create a short circuit (or shunt), a low-resistance path for current to flow, essentially bypassing the relay. The coils in the relay de-energize, closing the switch and lighting the LED to warn any nearby tiny drivers that a train is present on the tracks. It all depends on the train giving a preferential current path, which can be a problem if there are leaves or rust on the rails. You can see how shiny and clean tracks look when they’re in frequent use. Tracks that haven’t seen a train in a day or more often impose a speed restriction on the first train just in case there is rust that could affect the track circuits along the way.
If all this circuitry seems a little convoluted to simply detect the presence of a train, it’s because of how this simple track circuit handles when things go wrong. Let’s say the track circuit loses power; what happens? The relay deenergizes and falls back to the safest condition: assuming a train is occupying the tracks. Same thing if a rail cracks or breaks: the relay deenergizes and the light comes on. This is called failsafe operation, or as the engineers prefer to call it: fail to a known condition. If anything goes wrong, we want the default assumption to be that there’s a train coming because it might be true. Fail safe operation isn’t just in the track circuit but the warning devices too. Gates are actively held up with a powered brake. If power is lost, they fall just by gravity alone. And the bells and lights are usually powered by banks of batteries that can last for hours or days. Most modern train detection systems have moved to more sophisticated equipment, but relays are still used around the world because of their reliability. In fact, this is called a “vital” relay because of all the features that make it extremely unlikely to fail. You can see it acts slowly so that the inevitably noisy signal of a train shunting the tracks can’t cycle it on and off over and over; The armature assumes the de-energized position even if the spring breaks; The contacts use special materials to keep from welding together; And they’re just really robust and beefy to make sure they last for decades.
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