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Looking at the capacity of train networks, systems like CBTC help increase capacity, but the spacing of trains is ultimately limited by poor braking performance. In my understanding, safe distances are calculated by assuming a worst case situation ahead, such as a derailment or collision. The train behind must have adequate stopping distance plus a safety margin behind the train in front. Since trains have such poor braking performance, this required spacing is very large, especially for heavy and/or higher speed trains. Why aren't trains, especially in, for example, very dense subway systems, equipped with some auxiliary braking system, such as additional rubber shoes that could contact the track, or even maybe extreme things like plows off to the side of the track, that could stop the train very quickly in a rare emergency, such as the train ahead derailing?

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Let me add a bit more detail about the thinking behind this question: Commuter trains in my area can brake at about 0.05g, so they stop in about 1/2 mile. Once you multiply out all of the various safety factors, they wind up requiring 10 minute spacing. By comparison, a bus driving down the highway maintains a following distance of less than 10s. Since the spacing for trains is based on a worst case scenario of an instantaneous derailment ahead, and braking distance is proportional to the square of deceleration, even a 40% increase in emergency braking deceleration is good for nearly a 2x increase in capacity on a track. 0.1g is unlikely to cause injury to passengers, and this is only applicable in the worst case catastrophic derailment ahead. Since this scenario almost never happens, even a single use brake that completely destroys the train and damages the track is acceptable as long as the passengers aren't injured.

Another example in my area is BART. BART's bottleneck is in the transbay tube, with one track in each direction. There's been talk about building a second tube, perhaps at a roughly 10 billion dollar cost. This would be necessary because BART says it can't maintain a headway of less than 2.5 minutes safely. If a bus can maintain a 10s headway safely, the only thing keeping the capacity of BART from being 15x better is braking performance in an emergency.

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    $\begingroup$ Higher speed trains need to decelerate slowly, otherwise, the passengers will be injured or their baggage be damaged. $\endgroup$ – Jem Eripol Sep 27 '17 at 0:37
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    $\begingroup$ "Rubber shoes" would immediately liquefy before being able to impart any useful braking force. The kinetic energy (we're talking dozens/hundreds of megawatts of power to bring a train to a halt) needs to go somewhere - typically dissipated as heat in most braking systems. Alternatives such as eddy current braking are expensive and somewhat destructive to hardware. $\endgroup$ – user6335 Sep 27 '17 at 7:35
  • $\begingroup$ Yea, I can't imagine the rubber being used in steel tracks. Do trains use hydraulic brakes? $\endgroup$ – Jem Eripol Sep 27 '17 at 7:40
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    $\begingroup$ Tramway cars have electromagnetic emergency brakes - long bars that electromagnetically stick to rails creating huge friction. The forces in a train are a couple orders of magnitude higher though. $\endgroup$ – SF. Sep 27 '17 at 8:24
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    $\begingroup$ I think another issue here is that emergency systems are designed to prevent an accident in an emergency situation - routine operations should rely on the normal systems. If an emergency braking system is installed to allow trains to run at shorter intervals, then the system becomes reliant on the emergency system. $\endgroup$ – Mark Oct 2 '17 at 10:28
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Another contribution to the issue, if you want to stop a train of any length in a hurry you would need to uniformly brake every car in a controlled way. If you excessively brake the forward part you risk causing a derailment. Excessive braking at the rear also risks derailment if on any sort of curve.

This would seem to preclude any "simple minded" emergency plow/shoe type braking. So now you are left with fitting every car with a fairly sophisticated powered brake and control system. Feasible for passenger, but cost prohibitive for freight.

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  • $\begingroup$ It doesn't help that "feasible for passenger" doesn't necessarily mean "comfortable/survivable for passenger". Depending on how fast you intend to brake, it could be extremely violent and harmful to everyone inside. $\endgroup$ – JMac Sep 27 '17 at 16:06
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    $\begingroup$ @JMac I think you could comfortably break from subway speeds to zero in a lot less time than 2.5 minutes. $\endgroup$ – user253751 Dec 16 '19 at 14:36
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It's a boring answer, but the main reason is cost - in addition to the equipment itself there would need to be a maintenance schedule. There's also the question of additional mass.

Part of the quality/speed/cost triangle (https://en.wikipedia.org/wiki/Project_management_triangle). Additional emergency brakes are a high quality suggestion, but will increase time and cost for an item that in normal use will not be seen as necessary - it's usually only after a problem occurs that public expectations and legislation catch up with quality/safety recommendations.

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    $\begingroup$ Well, the argument for the brakes is not just about safety, but also increasing the profits through higher utilization of infrastructure: ability to squeeze more trains into a segment of railway by reducing distances between them. $\endgroup$ – SF. Sep 27 '17 at 8:26
  • $\begingroup$ I was picking up on the word "emergency". Cost is why people are reluctant to incorporate a feature that's normally not expected to be used. If the question was more about why trains don't run closer together there are other considerations to braking, such as signalling, point switching and platform availability at stations. $\endgroup$ – ItWasLikeThatWhenIGotHere Sep 27 '17 at 9:37
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    $\begingroup$ Well, in absence of the emergency feature, there's the emergency procedure, which is costing a fortune each day in keeping the trains waiting at choke points for the gap after the previous train to grow sufficiently long. $\endgroup$ – SF. Sep 27 '17 at 9:55
  • $\begingroup$ I'm not convinced. Even using the cost/benefit calculations used in The Fight Club, I suspect that cost takes a back seat to complexity, reliability, and the fact that braking at more than maybe 0.2g is likely to cause more injuries to the passengers than to whatever external injuries might result from most collision scenarios. $\endgroup$ – Carl Witthoft Sep 27 '17 at 17:19
  • $\begingroup$ @CarlWitthoft: Bulk of railway profit comes from cargo transport, and loaded cargo train cars are much more massive than passenger cars, while possessing the same braking force. Leave the old brakes and procedures for passenger cars, start implementing the new system in cargo trains and you're massively ahead profit-wise. Never mind, I've been on a train that underwent emergency braking. The deceleration could easily be quadrupled without significant risk of injury, and a crash usually results in multiple casualties. Some broken bones instead are a small price! $\endgroup$ – SF. Oct 12 '17 at 15:10
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I don't have time right now to research examples, but I have heard of many subway systems that have much better brakes than the cheap ones on freight trains. Since subways brake every few minutes to a full stop, that's a few hundred times per day and means that there is also a lot more maintenance to do. So separate disk brake disks are often used instead of brake shoes on the wheels like on freight trains.

I can also testify that acceleration and deceleration, for example on the Frankfurt U-Bahn is stronger than on other systems I have used. (Unfortunately, there are a few years between me in Frankfurt/Germany and me in San Francisco/USA, so I can't compare with DART.)

As for DART's headways, besides signalling and brakes, there might also changes to tracks be needed to withstand higher lateral forces. And as @agentp points out in his answers, braking of train cars has to be coordinated, so the train needs the required electronics to do that. Add in some other small things and it becomes so complex that you better get new trains. So smaller headways on subways (which unlike freight cars should be build with this in mind) are definitely possible, but adding them to an existing system is a major engineering project that requires long-term planning. (Munich S-Bahn did something like that successfully.)

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The complexity of the system is directly related to the length of the train. Any differential in the applied rate of deceleration among the cars accumulates in the couplers. The cars don't just have to apply the same braking effort, they have to actually each decelerate identically. This would require very intense load monitoring at the couplers and very fine and rapid response to a controller that can handle all varieties of cars and loads. And the system would have to be maintainable by the existing maintenance infrastucture. Have you ever been to a transit system maintenance yard? Do you think they will take a train out of service if one car has a brake fault?

An analogy would be water hammer. Try stopping the flow in a mile long pipe by shutting a valve quickly and see what happens.

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