The traditional problem of electrifying railroads is that the necessary overhead wires or third-rails cost several million dollars a mile, on the same order of cost as the track itself. American railroads have not been able to find this kind of money, as they are largely living on capital investments made circa 1910. Railroad investment largely ceased with the advent of the automobile and the airplane. However, if one recasts the problem of electrically-powered railroads in terms of electronically-switched power, certain possibilities open up. Let take three technologies. The first two have been reduced to practice as systems. The third, while speculative as a whole, does not have any unproven components.

1. The Alstom “smart” third-rail system described in the August 2015 issue of Trains. The Alstom system has been installed on streetcar lines in France, in short lengths through “historic districts” where poles and wires are objectionable, and most recently on the full length of a six-mile streetcar line in Dubai, in the Middle East, This system has a third-rail between the other two rails, and at the same height, but the third-rail has sections of insulator, breaking it up into segments of, say fifty feet, and each segment has an electronic switch, so it only becomes “hot” when commanded. The streetcar carries a shield to protect the activated segment from contact, and commands the third-rail segments to “light up” as appropriate. This system is rather more expensive than other methods of electrification, but can be used under circumstances which might preclude the use of these. In the long run, complex manufactured goods tend to become cheap, because they can be made in a factory, with suitable tools. Construction labor in the field remains persistently expensive, however.

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https://en.wikipedia.org/wiki/Ground-level_power_supply
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2. There are electric buses in China, which are powered by ultra-capacitors, and the capacitors are recharged by overhead wires which only exist at each bus stop. The capacitors only have to hold enough electricity to drive the bus for a quarter-mile or so, from one bus stop to the next, and a capacitor, unlike a battery, can recharge with extreme rapidity, in seconds. The cables at bus stops only cost a small fraction of what a continuous cable would cost. This system is of course extensible to trains. There is no reason why a railroad car cannot carry, say, a hundred tons of capacitors.

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http://www.slideshare.net/ResearchIndia/super-capacitor-buses-in-shanghai-5156990
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3. The John G. Kneiling Inter-Modal Platform, published in Trains back in the early 1980’s. This system consisted of a long articulated flatcar (I believe Kneiling stipulated a length of a thousand feet). The articulated car was to be fitted with many motors, a continuous on-board power cable, and power cars (locomotives) at either end. This was before double-stack cars became common, but the concept would be readily extended to them. While this system was never reduced to practice per se, it is nothing more than a “slug” locomotive on a grander scale. Trains normally run miles apart, and the cost of the on-board cable is small compared to that of electrifying trackage in the ordinary way. With suitable control electronics, with each truck motor receiving the optimum power/braking setting, the Intermodal Platform would have substantially the performance of a rail-car. Let us add to this, many pantographs or third-rail brushes, and a sufficiency of capacitors. The brushes or pantographs would retract when not in use, rather like a steam locomotive’s water-scoop.

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I’m afraid there is no easily accessible source for Kneiling, he predates the internet, and his material is all paywalled.

http://www.amazon.com/Integral-train-systems-John-Kneiling/dp/B0006CK4NU
http://cs.trains.com/trn/f/111/t/228469.aspx?page=1
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Taken in combination, electrified track would only need to have, say, a hundred or two hundred feet of Alstom “smart” third-rail, every half-mile or so. A short, light train (eg. a commuter train) would rely on its capacitors between third-rail segments. A long, heavy train would always have a brush on a third-rail segment, and would distribute the power from there, via its own internal cable. In many cases, a segment of “smart” third-rail would have its own capacitor, so that it could store electricity for an hour or more, accumulating it from the kind of electrical service which would be suitable for a discount store, and which is readily available where a county road crosses a railroad track, and then release it at a rate of 10,000 hp for a couple of minutes when a train comes by. It would be feasible to create “mini-helper districts,” say a thousand or two thousand feet long, with a higher proportion of third-rail, and the ability to transfer electricity from a train going down a grade (dynamic braking) to one going up the grade. In particular, commuter railroads often have short steep grades to achieve grade separation, and electric railcars are able to get up and down them without difficulty.

From the track point of view, the major expense would be that of building out electric grid connections to desolate locations on the main line, such as Abo Canyon in New Mexico.

From the rolling stock point of view, it would be necessary to dispense, once and for all, with idea that a railroad car is a sort of mobile warehouse, to be kept cheap and allowed to clog up the railroad. All classes of cars will have to develop systems for expeditiously loading and unloading, comparable to an inter-modal yard, or else these classes of cars will have to be superseded by specialized containers. For example, an articulated tank car would have many tanks, probably no more than twenty tons each, and a loading manifold, similar to the one in a tank-ship. Once connected to a terminal, it would open and close various valves and separately load and unload the contents of various tanks. Once cars achieved high utilization, it would become feasible to fit a comparatively small number of cars with an evolving standard of running gear.

There is a rather funny story about the “warehouse mentality” at its crudest, which I ran across in Edwin A. Pratt’s _The Rise of Rail Power in War and Conquest, 1833-1914_ ( P. S. King & Son, Ltd., London, 1915). It seems that during the Civil War, a Union Army paymaster, seeing an empty boxcar sitting on the main line, decided that it would make an admirable payroll office. So he moved in, and started paying the troops, and forbade anyone to move his “office.” This, of course, had the practical effect of throwing the railroad into gridlock.