Why are trains so heavy?
May 17, 2009 6:11 AM   Subscribe

Why are trains so heavy?

Train carriages (passenger cars) look very solidly built and look a lot heavier per person carried that a coach / bus. Is there a reason to build them so heavy?
posted by priorpark17 to Travel & Transportation (16 answers total) 2 users marked this as a favorite
 
To keep them on the rails
posted by mattoxic at 6:20 AM on May 17, 2009


Weight is indeed important. I was observing at the scene of a multi-car derailment and discovered to my surprise the cars are just sitting on top of the wheel assemblies with only a pin to hold them in place. They picked up the derailed cars, rolled in some new wheels and just dropped them on top.
posted by tommasz at 6:27 AM on May 17, 2009


Best answer: Also, unlike a bus, each train car has to withstand the pull of the rest of the train.
posted by sd at 6:42 AM on May 17, 2009 [1 favorite]


Response by poster: I think SD has it; the 1st passenger car is a link in a chain pulling the rest of the the train.
posted by priorpark17 at 6:58 AM on May 17, 2009


These days, they also have to be designed pretty strong to withstand crashes with other trains. Those other trains could include freight trains on many lines, which are obviously pretty darn heavy themselves.

The Federal Railroad Administration's standards in this area are a significant concern--the latest one I think requires passenger cars to withstand a 1 million pound buffering force, which is way more than it would experience just from pulling the attached cars.

For one thing is means the cars have to be built so heavy that they can't be articulated (one truck between cars like the TGV) and have to have a truck at each end of each car. Otherwise the load per axle is too high for the track. Perversely, this might actually have reduced safety, since articulated trains have historically fared quite well in derailments because they tend to stay in a straight line.
posted by FishBike at 7:17 AM on May 17, 2009


Using metal wheels on metal rails is one of the best ways to reduce rolling resistance, and thus make it easy to transport things. Unfortunately, when there's hardly any rolling resistance, there isn't hardly any grip to the rails either. And one way to overcome the problem that a cart could derail too easily because of this, you make it heavy. Because in mechanical engineering the best solution for one thing -- in this case rolling resistance -- always forces one to use suboptimal solutions for other problems. Which is a Law.
posted by ijsbrand at 7:27 AM on May 17, 2009


The other side of the argument is that they do it because they can- a bus needs to be able to navigate on roads with rubber tires. So they are made light enough to be able to do this efficiently. A train, with its steel wheels, doesn't need to be as light, so they don't waste any time making it any lighter. This saves them from making the engineering choices between reliability, strength, lifespan and lightness.
posted by gjc at 7:41 AM on May 17, 2009


And yet, take a look at some of the dearly departed rolling stock at nycsubway.org. These guys look like they're made of tin.
posted by thejoshu at 7:48 AM on May 17, 2009


You can always make stuff lighter by spending more money on lighter metals and composites. I don't know if the N700 has been made faster than predecessors by using lighter materials.

All French TGV weighs abut 1.0 or 1.1 tonne per seat. A car weights between 1.5 and 2 tons. So a fully loaded car is lighter per person than a fully loaded TGV, but TGVs usually run almost fully loaded, while a car normally supports only one person. So effectively the TGV weighs only half as much per person as a car.

Also, the TGV is a massive two story affair designed for sustained speeds of 300km/h (200mp/h) and includes 2 or 4 massive cars dedicated the engines. So a subway will weigh considerably less per "seat", although accounting for usage seems challenging. Btw, the TGV was designed in the 70s while modern cars use more modern materials.

As an aside, you must remember that trains & subways run off electrical wiring, alleviating the need for fuel, batteries, etc. 14 gallons of gas weighs about 90 pounds, or 1/20 tons, not enormous, but noticeable. If you convert this into energy, you must account for the energy used in obtaining, refining, and transporting that

I think an ideal solution to oil dependency is to build electrified highways, vaguely similar to the Ground-level power supply used in the Tramway de Bordeaux. Cars would use batteries for travel on surface streets, but recharge while traveling on the highways.
posted by jeffburdges at 7:55 AM on May 17, 2009


Surely if the weight was an end in itself they would just put in some concrete ballast instead of all that expensive steel.
posted by Rumple at 9:36 AM on May 17, 2009


Do remember that, apart from friction, keeping a heavy thing moving on a flat surface at a constant speed takes no more energy than keeping a light thing moving. It's only accelerating (or going up hills) that takes energy.

Of course, the devil is in the details, and that "apart from friction" is very important. If there's a lot of friction -- "rolling resistance" -- your vehicle will keep slowing down, and you'll need to provide energy in to keep speeding it back up again. The lighter your vehicle is, the less energy this will take. So for cars and busses, which have a lot of rolling resistance, you're better off energy-wise being lighter. But a train (according to ijsbrand) has low rolling resistance. That means that the train doesn't slow down as much due to friction, and it doesn't require so much energy to speed it back up again. Thus the energy cost of being heavy is much less for a train.
posted by wyzewoman at 10:10 AM on May 17, 2009


El trains have to be less heavy than their ground-bound cousins as thejoshu alluded to. Every ton of rolling stock on an El line has to have the tracks built to withstand that much more weight. It's a simple question of economics - a lighter train will require a less substantial elevated deck.
posted by JJ86 at 11:03 AM on May 17, 2009


Other stuff has to stop and start more often, which creates an incentive toward lightness.

Also, so as better to flatten pennies.
posted by Clyde Mnestra at 12:09 PM on May 17, 2009


You don't say where you are, but if you are in the US, FishBike is exactly right. The FRA has very strict regulations for trains that are on mixed-use lines, i.e., rails that share both freight and passenger trains. In those cases where there is mixed-use traffic, the passenger rail cars have to withstand a hypothetical collision with a freight train. This necessitates a much heavier rail car than would seem necessary. The Acela trains in the Northeast Corridor were burdened with this requirement, rather than being able to use lighter passenger rail cars in use in other countries.
posted by ambrosia at 12:29 PM on May 17, 2009


Best answer: Rail cars need to withstand forces during coupling and uncoupling, as well as just being pulled around, that would destroy a bus.

To couple rail cars (in the US), you essentially ram/bump them into each other at low speed. When buses do that, they call it a 'crash.' Railroads are typically pretty gentle during coupling/uncoupling when passengers are onboard, but the cars are designed for very rough handling.

Also, they are built to transmit force from one end of the car to the other, in a way a bus isn't. Think of a rail car that is located right behind the engine in a very long train; it is subjected to huge amounts of force which must be transmitted down the length of the car. This necessitates a very heavy frame.

Some of these constraints disappear if you're building cars for fixed-configuration trains, like the TGV or Shinkansen trainsets. If you're not going to couple and uncouple the cars, and if you always know the maximum length of the train and the forces each car needs to withstand, then you can design it with a lot less material. You don't have to assume and build it to withstand some sort of hypothetical worst-case scenario (like being car #1 in a 100-car mixed F/P train, or getting slammed around in a yard). This is why most of the high-speed trains in the world, and many subways, use fixed trainsets.

I am not familiar with the regulations, but I assume just based on the designs in use that in countries with high-speed trainsets the regulators have created different standards for them than would typically apply to reconfigurable cars and locomotives.
posted by Kadin2048 at 12:48 PM on May 17, 2009


US has regulations (and a general preference) for steel as they share the line with freight trains. At least, that's my understanding. The N700 run on special tracks, just like many other trains here.

Here is one of the newer models on the Odakyu line:
http://ja.wikipedia.org/wiki/%E5%B0%8F%E7%94%B0%E6%80%A550000%E5%BD%A2%E9%9B%BB%E8%BB%8A

Which weighs 260T for 358 people. About 700kg/person? If wikipedia is right :) One of the nicer trains to travel on btw..
posted by lundman at 6:10 PM on May 17, 2009


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