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.
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.
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.
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.
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.
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.
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.
Never mind. my comment was for main line tracks and not city rail.
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 (Note that it lists length of the lines as the #1 cause of power loss.)
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.
I wonder at the efficiency of generating electrical current with diesel motors to power the electric motors that drive the train.
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 ?
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.
Cool.
I guess one stops a steam locomotive by turning on the brakes and waiting.
And waiting.
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.
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?
To be fair to the OP, they weren’t asking why nobody thought of it before, they were wondering why it wasn’t done.
It costs less to ship by rail; the customer price is lower.
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?