# Economics of Train Length **URL:** \ **Category:** Factual Questions\ **Created:** [November 6, 2015, 3:46am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777 "2015-11-06T03:46:08Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Ornery\_Bob](https://avatars.discourse-cdn.com/v4/letter/o/da6949/32.png) [@Ornery\_Bob](https://boards.straightdope.com/u/Ornery_Bob)\ **Post date:** [November 6, 2015, 3:46am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/1 "2015-11-06T03:46:08Z")
My route home from work takes me down a street with a train crossing that gets a fair amount of container train traffic. Some of them are amazingly long even though I suspect they’re essentially “local” traffic (I live near a port) and there are longer ones out there. So why is it better to combine five locomotives to pull 500 cars than have the five locomotives pull 100 cars each? Either way the same number of cars get moved by the same number of locomotives. (numbers made up for illustrative purposes) The answer has to be that it’s more economical but how does that work? More cars gives you more momentum but also more friction. I’m also wondering what the limiting factor is for train length. What determines a train’s maximum length? Speaking of momentum, I suspect that all the stopping power also rests with the locomotives. Does it take as long to stop one of these behemoths as it does to get it up to speed? Thanks! ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [November 6, 2015, 4:02am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/2 "2015-11-06T04:02:07Z")
5 engines hooked together takes 1 crew 5 engines running separate takes 5 crews. Dynamic breaking is with the engines and has a limit. Each car has its own set of air breaks. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [November 6, 2015, 4:03am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/3 "2015-11-06T04:03:15Z")
2 things. 1)It’s possible they’re just moving some of the locomotives. They’re not operating, just being transported. 2)Each car has it’s own brake. The locomotives don’t (for the purposes of this discussion) provide the braking for the entire train, each car helps. 3) (okay, three things), Clearly it’s easier and cheaper to have one long train than 5 short ones running on one rail. The transport industry is, typically, pretty well tuned. Besides, if all the cars are going (more or less) to the same place, why not send them all at once rather than 5 trains, one at a time. ISTM, 5 separate trains on one track would be more of a headache, WRT logistics, than one. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [November 6, 2015, 4:05am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/4 "2015-11-06T04:05:28Z")
Those five locomotives are all controlled by a single engineer in the front one – the other ‘helper’ locos are slaved to that by remote control. So it saves a lot on salaries & benefits – only one crew is needed to run that train. Also, there are advantages in signaling, scheduling, safety etc. with one big train vs. 2 or 3 smaller ones, but those are less important. ---
**Author:** ![Tripler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripler/32/21807_2.png) [@Tripler](https://boards.straightdope.com/u/Tripler)\ **Post date:** [November 6, 2015, 4:07am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/5 "2015-11-06T04:07:16Z")
> [@Ornery\_Bob](#): > > Speaking of momentum, I suspect that all the stopping power also rests with the locomotives. Does it take as long to stop one of these behemoths as it does to get it up to speed? > > Thanks! If I recall a discussion correctly, I learned that each of the cars has a constant brake applied to its wheels. The locomotives power a compressed air system that holds the brake pads off of the wheels when the train is in motion. If the Engineer needs to stop quickly, he releases the pressure off that system, and the brake pads are reapplied to each car’s wheels. It’s a reverse system from your auto, which requires hydraulic pressure to apply brakes; trains hold brakes off with an active pneumatic system. Tripler 4 years of Engineering school, and they never let me play with the train. :mad: ---
**Author:** ![usedtobe](https://avatars.discourse-cdn.com/v4/letter/u/ec9cab/32.png) [@usedtobe](https://boards.straightdope.com/u/usedtobe)\ **Post date:** [November 6, 2015, 4:16am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/6 "2015-11-06T04:16:23Z")
Good Lawrd - Have these kids never heard of the “Westinghouse Air Brake”? Pls do your own copy/paste google. This is what: a. Made Westinghouse the name that it is b. Made long trains possible. (it works by using the brake line to keep the brakes OFF - if the line is cut or breaks, the brakes on each car engage - no more runaways.) ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [November 6, 2015, 4:21am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/7 "2015-11-06T04:21:19Z")
> [@usedtobe](#): > > (it works by using the brake line to keep the brakes OFF - if the line is cut or breaks, the brakes on each car engage - no more runaways.) I’ve heard of that. It totally ruined an episode of _Banacek_ for me. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [November 6, 2015, 5:35am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/8 "2015-11-06T05:35:51Z")
About a dozen of us moved a rail car full of LDPE about sixty feet closer to the plant. Got it started rolling with a large angle iron bar under a wheel, and the track was fairly level for those twenty yards. Not sure how we managed to do that with the brakes on. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [November 6, 2015, 6:19am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/9 "2015-11-06T06:19:24Z")
> [@usedtobe](#): > > Have these kids never heard of the “Westinghouse Air Brake”? > > (it works by using the brake line to keep the brakes OFF - if the line is cut or breaks, the brakes on each car engage - no more runaways.) You’d think these brakes would be used on all trains. Unfortunately they are not, as the town of Lac-Mégantic can testify. ---
**Author:** ![Ranger\_Jeff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ranger_jeff/32/2065_2.png) [@Ranger\_Jeff](https://boards.straightdope.com/u/Ranger_Jeff)\ **Post date:** [November 6, 2015, 8:07am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/10 "2015-11-06T08:07:11Z")
The parking brakes on a tractor trailer work similarly. With no air, a spring applies the brake. Apply air, and air goes into the upper brake chamber, pushes a diaphragm, compresses the spring and releases the wheel to turn. And in doing so, releases the lower brake chamber which the driver operates by stepping on or releasing the brake pedal. ---
**Author:** ![Chief\_Pedant](https://avatars.discourse-cdn.com/v4/letter/c/a6a055/32.png) [@Chief\_Pedant](https://boards.straightdope.com/u/Chief_Pedant)\ **Post date:** [November 6, 2015, 9:55am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/11 "2015-11-06T09:55:42Z")
> [@Ornery\_Bob](#): > > I’m also wondering what the limiting factor is for train length. What determines a train’s maximum length? Practical limitations like regulations, track (length of passing loop, e.g.), and type of railcar (braking system, e.g.) and so on. It’s not really a physical limitation, at least for trains with distributed locomotives, which can become quite long. Billiton’s record is over 4 miles long; almost 700 cars. But if you live where a train is holding up a crossing, you are not going to be happy with the time traffic is held up, so practical considerations drive length way before physical considerations do. ---
**Author:** ![SpyOne](https://avatars.discourse-cdn.com/v4/letter/s/82dd89/32.png) [@SpyOne](https://boards.straightdope.com/u/SpyOne)\ **Post date:** [November 6, 2015, 10:43am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/12 "2015-11-06T10:43:36Z")
> [@Ornery\_Bob](#): > > So why is it better to combine five locomotives to pull 500 cars than have the five locomotives pull 100 cars each? Either way the same number of cars get moved by the same number of locomotives. (numbers made up for illustrative purposes) Well, they’ve been doing multi-locomotive trains for over a hundred years, and this only applies recently, but: on modern trains, you can link the controls of multiple locomotives together. So 5 locomotives essentially become 1 locomotive with 5x the power. And the savings there is only one driver, instead of 5. > [@](#): > > I’m also wondering what the limiting factor is for train length. What determines a train’s maximum length? > > Speaking of momentum, I suspect that all the stopping power also rests with the locomotives. Does it take as long to stop one of these behemoths as it does to get it up to speed? Each car has its own brakes, controlled from the locomotive. In fact, trains apply the brakes in the rear cars first and then work their way forward. This way each car is dragging the ones in front of it. Doing it the other way results in each car getting hit by the weight of all the cars behind it. So there you have a limiter on the length of trains: braking. Longer trains need more time before they have the brakes on on all the cars, which means they need longer distances to stop. (Very short trains, heck just a locomotive, will still need a really big space to come to a stop, but longer trains need even more.) But I suspect the biggest trains you’ll ever see aren’t at or near the maximum length for safe braking. See, at both the beginning and the end (and often occasionally in the middle) of it’s journey, a train is a “yard”, where multiple tracks make it easy to add and remove cars from a train. There are also plenty of side-by-side tracks, so a train can pass through yard while other trains are being assembled or disassembled on parallel tracks. If you make a train longer than the side-tracks, it will eventually be blocking the entrance or the exit. Even if their are parallel tracks coming into and out of the yard, eventually the train is blocking one of them. So at some point a train becomes too big to really fit in a yard. And that causes problems. You’d think they’d just build the yards to accommodate the longest possible train, But land isn’t free. And some of those yards were built a hundred years ago. so the yards are the biggest they could be made, economically, given a lot of factors that have nothing to do with trains. And then the biggest a train can be is the biggest that will fit properly into the yards it needs to fit into. ---
**Author:** ![SpyOne](https://avatars.discourse-cdn.com/v4/letter/s/82dd89/32.png) [@SpyOne](https://boards.straightdope.com/u/SpyOne)\ **Post date:** [November 6, 2015, 10:54am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/13 "2015-11-06T10:54:26Z")
One other reason for multiple locomotives that probably doesn’t apply to your location: sometimes a train needs more power for a short portion of its journey. I once read a great day-in-the-life story about an engineer who lived in California, and he and his locomotive would spend the day helping trains get over the Rockies. Start with one going east, but unhook as soon as they were out of the mountains and hook onto a westbound one. Back and forth over the same piece of track all day long. The story was about one day where various things conspired to make the last run of the day hell. It was still the Age of Steam (may have been the 1940s), he had one of those engines where they mounted the cab right up front (which were better for the engineer in tunnels), but then the turntable broke and he couldn’t get turned around, and he had to be attached to the rear of the train instead of the front, so every single tunnel he entered was already filled with smoke from all the locomotives, including his own. The other drivers pushed the speed limits so he’d spend as little time in the tunnels as possible, but … Occasionally I remind myself that the worst day I ever spent at work did not have a particularly worrisome chance of my suffocating, and so it really isn’t so bad. ---
**Author:** ![SpyOne](https://avatars.discourse-cdn.com/v4/letter/s/82dd89/32.png) [@SpyOne](https://boards.straightdope.com/u/SpyOne)\ **Post date:** [November 6, 2015, 10:58am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/14 "2015-11-06T10:58:19Z")
> [@SpyOne](#): > > See, at both the beginning and the end (and often occasionally in the middle) of it’s journey, a train is a “yard”,… **IN** a yard. Sorry. > [@](#): > > Even if their are parallel tracks coming into and out of the yard, … And of course “there”, not “their”. Not sure how I botched that one. ---
**Author:** ![Grumman](https://avatars.discourse-cdn.com/v4/letter/g/43a26b/32.png) [@Grumman](https://boards.straightdope.com/u/Grumman)\ **Post date:** [November 6, 2015, 11:28am UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/15 "2015-11-06T11:28:58Z")
> [@eschereal](#): > > About a dozen of us moved a rail car full of LDPE about sixty feet closer to the plant. Got it started rolling with a large angle iron bar under a wheel, and the track was fairly level for those twenty yards. Not sure how we managed to do that with the brakes on. If the brakes work on the difference between the reservoir pressure and the train line pressure, could it have been that the reservoir was empty and thus the brakes were off even though the train line was also empty? ---
**Author:** ![Muffin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/muffin/32/4341_2.png) [@Muffin](https://boards.straightdope.com/u/Muffin)\ **Post date:** [November 6, 2015, 12:09pm UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/16 "2015-11-06T12:09:19Z")
> [@Tripler](#): > > Tripler > 4 years of Engineering school, and they never let me play with the train. :mad: Looks like your luck was taken by a couple of my friends. One knows nothing about trains other than that it is a good idea to stop at railway crossings when a train is approaching. He got to drive a train at work in the company yard when they were short a person – the only instruction he was given was how to get it going, control its speed, and stop it. Another friend, years previously, and also with no training, was occasionally asked to drive a train in the company yard, but on one occasion he messed up and derailed some cars. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [November 6, 2015, 12:40pm UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/17 "2015-11-06T12:40:27Z")
> [@Grumman](#): > > If the brakes work on the difference between the reservoir pressure and the train line pressure, could it have been that the reservoir was empty and thus the brakes were off even though the train line was also empty? That’s the opposite of how it works. Pressure keeps the brakes off. It’s designed as a fail-safe. If a line ruptures, the train stops. If part of the train where to disconnect, that section would stop instead of continuing down the line on it’s own, etc. As for how they pushed it, either someone pumped up the system in that car beforehand or there’s a manual release lever of some kind. My vote is for the former. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [November 6, 2015, 1:00pm UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/18 "2015-11-06T13:00:26Z")
> [@Joey\_P](#): > > That’s the opposite of how it works. Pressure keeps the brakes off. It’s designed as a fail-safe. If a line ruptures, the train stops. If part of the train where to disconnect, that section would stop instead of continuing down the line on it’s own, etc. **Grumman** is correct. The [Westinghouse air brake](https://en.wikipedia.org/wiki/Railway_air_brake#Westinghouse_air_brake) still relies on an onboard air reservoir to apply the brake - i.e. that’s what provides the force to apply the brake. When the brake line is pressurized, this disengages the brake, and also tops off the reservoir. When the brake line pressure drops, the air in the reservoir pushes on the brake shoes. But if the railcar is left alone for a long time, the air in the reservoir leaks away, and the brake no longer works. For this reason, rail cars also have hand brakes. But those are not as powerful as the pneumatic brakes, and not always used. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [November 6, 2015, 1:09pm UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/19 "2015-11-06T13:09:10Z")
> [@dtilque](#): > > You’d think these brakes would be used on all trains. Unfortunately they are not, as the town of Lac-Mégantic can testify. That train did have Westinghouse style brakes. But as I explained in the above post, the [Westinghouse brake](https://en.wikipedia.org/wiki/Railway_air_brake#Westinghouse_air_brake) relies on onboard air reservoirs to activate the brake. The reservoirs are topped off by the brake line air pressure. When the train is left parked with the locomotives shut down, the air reservoirs leak out, and the brakes eventually disengage. In the [Luc Megantic disaster](https://en.wikipedia.org/wiki/Lac-M%C3%A9gantic_rail_disaster), the engineer did leave one locomotive running for this reason, but the train was left unattended. And the locomotive caught fire. Fire department came, shut down the locomotive and put out the fire, but didn’t call the engineers. And there weren’t enough hand brakes engaged to keep the train from moving down the slight incline. ---
**Author:** ![Muffin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/muffin/32/4341_2.png) [@Muffin](https://boards.straightdope.com/u/Muffin)\ **Post date:** [November 6, 2015, 1:10pm UTC](https://boards.straightdope.com/t/economics-of-train-length/736777/20 "2015-11-06T13:10:45Z")
Bingo. Here’s a link to the ensuing investigation. > [@](#): > > \*\*[Lac-Mégantic runaway train and derailment investigation summary](http://www.tsb.gc.ca/eng/rapports-reports/rail/2013/r13d0054/r13d0054-r-es.asp) > \*\* > . . . . > > Once the fire was extinguished, the firefighters and the track foreman discussed the train’s condition with the rail traffic controller in Farnham, and departed soon afterward. With all the locomotives shut down, the air compressor no longer supplied air to the air brake system. As air leaked from the brake system, the main air reservoirs were slowly depleted, gradually reducing the effectiveness of the locomotive air brakes. Just before 1 a.m., the air pressure had dropped to a point at which the combination of locomotive air brakes and hand brakes could no longer hold the train, and it began to roll downhill toward Lac-Mégantic, just over seven miles away. > > As it moved down the grade, the train picked up speed, reaching a top speed of 65 mph. It derailed near the centre of the town at about 1:15 a.m. > > . . . . > > Almost all of the 63 derailed tank cars were damaged, and many had large breaches. About six million litres of petroleum crude oil was quickly released. The fire began almost immediately, and the ensuing blaze and explosions left 47 people dead. Another 2000 people were forced from their homes, and much of the downtown core was destroyed. > > . . . . > > **Air brakes 101** > > Trains have two types of air brakes: automatic brakes and independent brakes. > > Automatic air brakes are used to slow or stop the entire train, and are controlled by means of a brake pipe connected to each car and locomotive. Decreases in pressure within this pipe cause air to flow into each car’s control valve, which injects stored air into the brake cylinder, applying the brake shoes to the wheels. > > By contrast, independent air brakes are available only on locomotives. They are activated by the direct injection of air into their brake cylinders, which then apply the brake shoes to the wheels. > > Both independent brakes and automatic brakes are supplied with air from a compressor on each locomotive. When a locomotive is shut off, the compressor no longer supplies the system with air. > > When air leaks from the various components, the pressure in the brake cylinders gradually drops, and the amount of force being applied to the locomotive wheels by the independent brakes is reduced. Eventually, if the system is not recharged with air, the brakes will become ineffective and provide no braking force. > > When the air brake control valves sense a drop in pressure in the brake pipe, they are designed to activate the brakes on each car. In this accident, however, the rate of leakage was slow and steady—approximately 1 pound per square inch per minute—and so the automatic brakes did not apply. > > [**Diagram**](http://www.tsb.gc.ca/eng/rapports-reports/rail/2013/r13d0054/Images/r13d0054-figure-04.png) [Next page](https://boards.straightdope.com/t/economics-of-train-length/736777.md?page=2)