Retrotechtacular: The Power To Stop

In everyday life, the largest moving object most people are likely to encounter is probably a train. Watching a train rolling along a track, it’s hard not to be impressed with the vast amount of power needed to put what might be a mile-long string of hopper cars carrying megatons of freight into motion.

But it’s the other side of that coin — the engineering needed to keep that train under control and eventually get it to stop — that’s the subject of this gem from British Transport Films on “The Power to Stop.” On the face of it, stopping a train isn’t exactly high-technology; the technique of pressing cast-iron brake shoes against the wheels was largely unchanged in the 100 years prior to the making of this 1979 film. The interesting thing here is the discovery that the metallurgy of the iron used for brakes has a huge impact on braking efficiency and safety. And given that British Railways was going through about 3.5 million brake shoes a year at the time, anything that could make them last even a little longer could result in significant savings.

It was the safety of railway brakes, though, that led to research into how they can be improved. Noting that cast iron is brittle, prone to rapid wear, and liable to create showers of dangerous sparks, the research arm of British Railways undertook a study of the phosphorus content of the cast iron, to find the best mix for the job. They turned to an impressively energetic brake dynamometer for their tests, where it turned out that increasing the amount of the trace element greatly reduced wear and sparking while reducing braking times.

Although we’re all for safety, we have to admit that some of the rooster-tails of sparks thrown off by the low-phosphorus shoes were pretty spectacular. Still, it’s interesting to see just how much thought and effort went into optimizing something so seemingly simple. Think about that the next time you watch a train go by.

22 thoughts on “Retrotechtacular: The Power To Stop

  1. “In everyday life, the largest moving object most people are likely to encounter is probably a train.”
    Largest manmade object, the Sun is moving with respect to the Earth. (Even larger are some stars although the great distance means they appear to be tiny and moving extremely slowly.)

    1. I think the intent was “likely to think about as a powerful moving object”. Most people don’t sit around thinking about how much energy would be required to stop the sun from moving.

      1. Zero,no energy at all. Just make it the reference. There is no reason you can’t as there is no absolute reference in the universe and in out “solar” system it’s already not moving.

    2. If you encounter (unexpectedly experience or be faced with) the Sun you were too close. However, anyone were barges and ocean going ships are will see trains as rather small; see “How to Avoid Huge Ships” by John W. Trimmer. It’s a bargain in paperback going for roughly $300 from some sellers.

      1. This is true. Even a cruise ship is much heavier than a freight train. Supertankers are much larger again.

        It is said that none can even compare to lighthouses, though they refuse to move.

  2. A surprising amount of trains don’t just waste their prior effort by turning it all to heat, but instead uses regenerative breaking.

    And regenerative breaking doesn’t really require that the train is grid connected either. Since on board energy storage in the form of either a flywheel, super capacitor bank, or just batteries is all alternatives when access to a grid isn’t an option, all though not that practical for longer freight trains.

    Regenerative breaking after all both reduces the wear on break pads, but also reduces fuel/energy consumption, and thereby a bit of the overall running cost. Though the added complexity of energy storage/transfer has its own costs associated with it and making it debatable if and when it is practical to use.

      1. I’m not aware of resistor-braking being very common among UK locos, but even if so fitted, they’d still be using air-brake systems to actuate the mechanical brakes on the carriages/wagons they’re hauling.

        Some diesel-electric multiple units (Class 220/221/222 Voyagers) are fitted with rheostatic brakes, but they’re not an option on the more numerous diesel-mechanical / diesel-hydraulic units.

        A more pressing issue (if you’ll forgive the pun) for British railways since this film was made, is the downsides of switching from tread brakes to disc brakes. The film mentions that the (then-new) High Speed Trains were fitted new, more efficient disc brakes, but also had tread brakes in order to scrub the wheels clean and improve adhesion. In the subsequent decades, as tread brakes fell out of fashion, we found that disc-brake equipped trains suffered from more wheel-slip issues, and intermittent detection by signalling track circuits. (Think of leaves on the line, pulverized by passing trains.)

  3. also interesting is the technology how to tell the brakes to apply and loosen again with pressurized air: due to the speed of sound in the brake pipe the cars in the back start to brake later, which is a problem on long trains, so you have to slow down the applying of brake power. Here[tm] is a lever on the cars (P — “Personenzug” for fast rise time on short trains, G — “Güterzug” for slow rise time on long trains), which is set accordingly. The same applies to the end of the braking: at first the train doesn’t brake, but when it brakes, it doesn’t stop braking, especially on trains “in G”.
    Next problem on long trains is the detecting of the beginning of the braking process in the rear part, the initial pulse loses its steepness on the way through the brake pipe. So the valves have a small chamber, which is filled, when the valve starts to apply the brake, and emptied when the brake is loose again. That keeps the steepness, so all the valves in the train detect the braking (slow changes of pressure are used to set the initial pressure of the brake pipe before departure).
    On some fast trains here[tm], there is a “Schnellbremsverstärker”, which detects a sudden pressure loss and removes additional air from the brake pipe to assist the braking, but this only works properly, if it is present and working in most of the cars in the train.
    On really long trains (like in the US) an additional problem on the end of the braking is, that it takes very long to refill the brake pipe throughout the train to the original pressure, so there the brakes are “einlösig”. That means, any increase in pressure loosens the brake completely, and on the next brake event you have to lower the pressure even more before the brakes apply. You can do that two or three times, but at least then you have to refill the brake pipe completely to get back the full braking capacity. Dynamically braking helps with that, so you don’t need to stop for refilling, but can regulate the downhill speed without applying the air brake too often in short time. On US trains there is some valve like “our” Schnellbremsverstärker, too, but for the regular braking, so the air on brake-apply does not have to move through all the train to the locomotive to escape. For this to work properly, there must not be more than two or three cars without working brake consecutive in train, otherwise the next valve might not detect the braking.
    The opposite of “einlösig” is “mehrlösig”, which means, a slight rise in pressure will loosen the brake a little, so you can regulate the brake power more easy, and you don’t lose the ability to brake at all when braking multiple times in short order, but you have to refill the brake pipe completely, before all the brakes are loose again. So when you have a whimsy compressor on your locomotive, you have to drive like the train was einlösig.

    Oh, and for safety reasons the loose state is with pressure in the brake pipe, and brake state is lower or no pressure (I forgot to mention earlier).

  4. Also worth thinking about: what happens when train breaks fail.

    47 dead, most of a small town incinerated on a busy summer Saturday night (2013)
    Single train operator, locomotives turned off, except for one (which lit on fire), slight grade. The air pressure slowly drained, the air breaks loosened without kicking on the emergency air breaks. The few hand breaks the single operator had set failed to keep it stopped. It rolled downhill (lights off, at 65 mph in a 10 mph zone), derailed, and ignited 72 cars of oil in the middle of town on a busy Saturday night. Fascinating and terrifying.

    1. I still find it incredible that stuff like this can happen – it seems so easy to imagine very basic systems for signals etc. that automaticlaly trip an emergency brake and yet it doesn’t seem to be a common feature.

  5. JohnU: it is a common feature, it works by emptying the main brake pipe (which is what you do to apply all the available brake force), but if the pipe is already empty due to leaks … this is why you have to apply hand brakes or other means to secure cars/trains, if they are standing for more than one hour without air supply. How many of it you need regulated and depends on the grade where the cars stand. The backup safety mechanism mentioned in the wiki-article may be a switch to a dead end just below the train parking lot, but since these are expensive in installing and maintenance, they are only on places, where cars/trains park on a regular basis and/or where a runaway car may cause major trouble (well, at least here[tm], other countries may have different rules).
    The cheap version of the dead-end-switch is a derailment device (“Gleissperre”), but for obvious reasons these are only allowed in tracks without passing trains (“Hauptgleis” vs. “Nebengleis”, in which only shunting happens — the train of the Lac Megantic incident stood in the canadian equivalent of a “Hauptgleis”, if I understand correctly, where normally no longer parking occurs). Main difference between shunting and moving a train is the signalling: on shunting you have to watch out for all and everything (“Rangierfahrt”), and as train (“Zugfahrt”) you only have to watch the signals, which indicate the speed which is safe to go (and at which you have no chance to stop in time anyways, if you see something in front of you).
    (disclaimer: this is only the general picture, there are a lot more details in reality)

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