# Why couldn't trains draw power from the railroad tracks? **URL:** \ **Category:** Factual Questions\ **Created:** [June 13, 2014, 5:54pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717 "2014-06-13T17:54:58Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![dstarfire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dstarfire/32/5762_2.png) [@dstarfire](https://boards.straightdope.com/u/dstarfire)\ **Post date:** [June 13, 2014, 5:54pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/1 "2014-06-13T17:54:58Z")
A lot of city buses, subways, and the like draw power from overhead lines or adjacent rails. Since train rails and wheels are metal why couldn’t you send power to a locomotive through the tracks? Or, better yet, draw power from trains during braking (when the motors become generators)? I imagine you could have links to the local power grid (through a transformer) every so often to reduce losses. ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [June 13, 2014, 5:59pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/2 "2014-06-13T17:59:30Z")
> **[Third rail](https://en.wikipedia.org/wiki/Third_rail)** > > A third rail, also known as a live rail, electric rail or conductor rail, is a method of providing electric power to a railway locomotive or train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track. It is used typically in a mass transit or rapid transit system, which has alignments in its own corridors, fully or almost fully segregated from the outside environment. Third rail systems are usually supplied from direct current electricity. > Modern ... Third Rail systems are the closest to what you have in mind. They don’t use the running rails for a variety of reasons but the first systems apparently did. ---
**Author:** ![Sicks\_Ate](https://avatars.discourse-cdn.com/v4/letter/s/258eb7/32.png) [@Sicks\_Ate](https://boards.straightdope.com/u/Sicks_Ate)\ **Post date:** [June 13, 2014, 6:07pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/3 "2014-06-13T18:07:45Z")
If you didn’t fence it off, you’d constantly have trains running over the bloated carcasses of dead deer, skunks, possums, raccoons, squirrels, rabbits, chipmunks, bears, etc. etc. that had been electrocuted. Diesel locomotives are extremely efficient…It’s probably simply not efficient to build another infrastructure for power delivery to trains. ---
**Author:** ![Saltire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saltire/32/553_2.png) [@Saltire](https://boards.straightdope.com/u/Saltire)\ **Post date:** [June 13, 2014, 6:08pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/4 "2014-06-13T18:08:14Z")
The two biggest reasons, I think, are: 1)People could get hurt. Moving a train takes a lot of power, enough to easily kill people. Having that power well out of people’s reach makes it safer. 1. It would cost more. Train rails aren’t put together to have good electrical connections betweent he segments. To run electricity through them, you’d need to change the way they were constructed in some way, such as putting a copper bridge between each pair of rails. Overhead, you can just run a cable, which is cheaper and can be spliced well to go between lengths. ---
**Author:** ![Saltire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saltire/32/553_2.png) [@Saltire](https://boards.straightdope.com/u/Saltire)\ **Post date:** [June 13, 2014, 6:15pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/5 "2014-06-13T18:15:21Z")
Just realized I may have misunderstood your question. I thought you ware asking why trolleys and such use overhead wires instead of electrified rails. But you probably were asking about electric locomotives to replace diesel-electric ones commonly used. The same things apply, except to say that the power needs would be much higher. The rails would have to be running so much juice that you couldn’t rely on air as an insulator. Power would leak into the ground all along the line. Something I should have put in the first post: Steel isn’t the best electrical conductor. You’d have far more resistance than in a copper cable, which would cause losses and heating in the rails. So the tracks would be hot in two senses, and you’d waste a ridiculous amount of energy. ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [June 13, 2014, 6:20pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/6 "2014-06-13T18:20:04Z")
> [@Saltire](#): > > 1. It would cost more. Train rails aren’t put together to have good electrical connections betweent he segments. To run electricity through them, you’d need to change the way they were constructed in some way, such as putting a copper bridge between each pair of rails. Overhead, you can just run a cable, which is cheaper and can be spliced well to go between lengths. Not to mention that train rails need to have gaps for thermal expansion, so even if there was a copper bridge connecting them, that bridge would be changing shape seasonally as the gap changed size. Eventually one of them would wear out and break, and the circuit would depend on every single bridge being intact. An overhead wire adjusts to temperature by the slack in the line changing, which is much more robust, but infeasible for rails. ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [June 13, 2014, 6:21pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/7 "2014-06-13T18:21:54Z")
> [@Saltire](#): > > 1. It would cost more. Train rails aren’t put together to have good electrical connections betweent he segments. Actually they are - not for power, but for the signalling system, which relies on detecting train axles by completing a circuit across the running rails. Depending on the system, one or both of the running rails may also be used as a ground for the third rail. This is the case for all the third rail systems in the NYC area, including the NYC Subway, Metro-North (except the New Haven line) and the Long Island Railroad. Third rails carry the motive power because they can be elevated a few inches and set off from the running rails. They typically run about 600-700 volts with very high current. Overheard caternary wires can run at much higher voltages (typically around 12-13kV on the New Haven/Northeast Corridor) because they are further from the ground. ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [June 13, 2014, 6:38pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/8 "2014-06-13T18:38:29Z")
Never mind. my comment was for main line tracks and not city rail. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [June 13, 2014, 6:48pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/9 "2014-06-13T18:48:46Z")
An additional issue is transmission loss over the power lines. Low-voltage lines tend to lose power to heat relatively quickly - not fast enough to be a problem in the tens of miles covered by a city, but enough to be a problem in the hundreds or thousands of miles covered by tracks. High-voltage transmission lines have less loss over distance, but then you need to step that power down to usable voltages. Thicker transmission lines also have less loss, but then you’re talking about a more expensive wire in the construction phase. So you have a lot of trade-offs that ultimately make a diesel generator look attractive. Here’s a good link talking about electrical transmission losses in technical terms: [http://electrical-engineering-portal.com/total-losses-in-power-distribution-and-transmission-lines-1](http://electrical-engineering-portal.com/total-losses-in-power-distribution-and-transmission-lines-1) (Note that it lists length of the lines as the #1 cause of power loss.) ---
**Author:** ![drachillix](https://avatars.discourse-cdn.com/v4/letter/d/48db29/32.png) [@drachillix](https://boards.straightdope.com/u/drachillix)\ **Post date:** [June 13, 2014, 6:52pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/10 "2014-06-13T18:52:17Z")
> [@Sicks\_Ate](#): > > Diesel locomotives are extremely efficient…It’s probably simply not efficient to build another infrastructure for power delivery to trains. This bears repeating, trains are incredibly efficient, so much so that trucking companies love loading trailers onto flatbed rail cars to haul across the country if feasible. Many trucking company contracts specifically forbid this practice so they know that customers know are paying for a truck the whole way. > **[Intermodal freight transport](https://en.wikipedia.org/wiki/Intermodal_freight_transport)** > > Intermodal freight transport involves the transportation of freight in an intermodal container or vehicle, using multiple modes of transportation (e.g., rail, ship, aircraft, and truck), without any handling of the freight itself when changing modes. The method reduces cargo handling, and so improves security, reduces damage and loss, and allows freight to be transported faster. Reduced costs over road trucking is the key benefit for inter-continental use. This may be offset by reduced timings fo... ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [June 13, 2014, 7:36pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/11 "2014-06-13T19:36:55Z")
> [@drachillix](#): > > This bears repeating, trains are incredibly efficient, so much so that trucking companies love loading trailers onto flatbed rail cars to haul across the country if feasible. Many trucking company contracts specifically forbid this practice so they know that customers know are paying for a truck the whole way. > > [Intermodal freight transport - Wikipedia](http://en.wikipedia.org/wiki/Intermodal_freight_transport) I wonder at the efficiency of generating electrical current with diesel motors to power the electric motors that drive the train. ---
**Author:** ![davida03801](https://avatars.discourse-cdn.com/v4/letter/d/b5e925/32.png) [@davida03801](https://boards.straightdope.com/u/davida03801)\ **Post date:** [June 13, 2014, 7:41pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/12 "2014-06-13T19:41:04Z")
As noted previously is called the Third Rail. Only used in mass transit systems where there is a dedicated corridor that limits access to the very high voltage needed to power the electric traction power. Also noted for several reasons cannot be used elsewhere for practical maintenance reasons and immediate death to anything that touches the 3rd rail. You don’t think all of this was considered by the rail designers of generations ago ? ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [June 13, 2014, 7:55pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/13 "2014-06-13T19:55:36Z")
> [@carnivorousplant](#): > > I wonder at the efficiency of generating electrical current with diesel motors to power the electric motors that drive the train. AIUI it compares pretty well with the alternative of using a clutch and gearbox, which would obviously have to be massively strong and has inefficiency losses all of its own. Also with a diesel-electric you get to use regenerative braking. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [June 13, 2014, 8:08pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/14 "2014-06-13T20:08:19Z")
> [@Malacandra](#): > > AIUI it compares pretty well with the alternative of using a clutch and gearbox, which would obviously have to be massively strong and has inefficiency losses all of its own. Also with a diesel-electric you get to use regenerative braking. > [@](#): > > From Wikipedia: During braking, the traction motor connections are altered to turn them into electrical generators. The motor fields are connected across the main traction generator (MG) and the motor armatures are connected across the load. The MG now excites the motor fields. The rolling locomotive or multiple unit wheels turn the motor armatures, and the motors act as generators, either sending the generated current through onboard resistors (dynamic braking) or back into the supply (regenerative braking). Compared to electro-pneumatic friction brakes, braking with the traction motors can be regulated faster improving the performance of wheel slide protection. > > For a given direction of travel, current flow through the motor armatures during braking will be opposite to that during motoring. Therefore, the motor exerts torque in a direction that is opposite from the rolling direction. Cool. I guess one stops a steam locomotive by turning on the brakes and waiting. And waiting. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [June 13, 2014, 8:13pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/15 "2014-06-13T20:13:48Z")
> [@carnivorousplant](#): > > I wonder at the efficiency of generating electrical current with diesel motors to power the electric motors that drive the train. It’s pretty efficient. One of the best things about it is that you can run the diesel all the time at it’s most efficient RPM and torque, which makes a big big difference for efficiency. Meanwhile the the electric motors are just as happy barely moving and putting out tremendous torque to get things moving as they are spinning away at 60 mph. This is a big part of why hybrid autos get such good gas milage, and it’s even more significant for locomotives which have to push a lot harder to get things moving. ---
**Author:** ![MichaelEmouse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/michaelemouse/32/101_2.png) [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)\ **Post date:** [June 13, 2014, 8:15pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/16 "2014-06-13T20:15:06Z")
> [@drachillix](#): > > This bears repeating, trains are incredibly efficient, so much so that trucking companies love loading trailers onto flatbed rail cars to haul across the country if feasible. Many trucking company contracts specifically forbid this practice so they know that customers know are paying for a truck the whole way. > > [Intermodal freight transport - Wikipedia](http://en.wikipedia.org/wiki/Intermodal_freight_transport) Why would customers care if it gets shipped by truck or train? As long as it gets from point A to point B at time T in condition C for price P, why would customers insist on using trucks all the way? ---
**Author:** ![phreesh](https://avatars.discourse-cdn.com/v4/letter/p/9fc29f/32.png) [@phreesh](https://boards.straightdope.com/u/phreesh)\ **Post date:** [June 13, 2014, 8:21pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/17 "2014-06-13T20:21:21Z")
> [@davida03801](#): > > You don’t think all of this was considered by the rail designers of generations ago ? To be fair to the OP, they weren’t asking why nobody thought of it before, they were wondering why it wasn’t done. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [June 13, 2014, 8:34pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/18 "2014-06-13T20:34:35Z")
> [@MichaelEmouse](#): > > Why would customers care if it gets shipped by truck or train? As long as it gets from point A to point B at time T in condition C for price P, why would customers insist on using trucks all the way? It costs less to ship by rail; the customer price is lower. ---
**Author:** ![aldiboronti](https://avatars.discourse-cdn.com/v4/letter/a/9fc348/32.png) [@aldiboronti](https://boards.straightdope.com/u/aldiboronti)\ **Post date:** [June 13, 2014, 8:44pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/19 "2014-06-13T20:44:42Z")
The Southern Region of British Rail was electrified using the third rail system after the Second World War. I live in Portsmouth and all the mainline trains to London, Southampton, Brighton, etc used (and still use, as far as I know) the electrified third rail. I can’t recall many accidents involving electrification. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [June 13, 2014, 9:01pm UTC](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717/20 "2014-06-13T21:01:48Z")
> [@aldiboronti](#): > > The Southern Region of British Rail was electrified using the third rail system after the Second World War. I live in Portsmouth and all the mainline trains to London, Southampton, Brighton, etc used (and still use, as far as I know) the electrified third rail. I can’t recall many accidents involving electrification. I wonder what the distances involved are, and at what distances the rails are powered. Japan also has electric commuter trains, does it not? [Next page](https://boards.straightdope.com/t/why-couldnt-trains-draw-power-from-the-railroad-tracks/690717.md?page=2)