Max: agree that electrics are attractive in high-frequency or heavy-haul environments. But in the US (and Canada) high enough frequencies are few and far between, and heavy haul is already handled well by modern diesels. Also: US railroads buy locomotives that can run pretty much anywhere; the power tied on to a container train in Long Beach will likely stay there to its destination, whether Chicago/Texas/Atlanta/New York, but can also be used on any other part of the affected railroad(s). Very Clear Point: US railroads do not change power on trains, if at all possible, between origin and destination, which are typically 2000 or more miles (3000+ km) apart. This is quite different from freight operations in most other parts of the world.

The helper/braking helper is routinely done in the US already with diesels. Practically all long-distance or heavy trains have distributed power, with at least a trailing unit if not mid-train as well, that participate in dynamic braking as well as producing power. The main benefit in this process from electric would be the ability to regenerate - but that also depends on having enough nearby traffic to absorb the power produced, or a place to store it, neither of which is common on US railroads.

Then there's the capital cost of the electrification and the new locomotives. Most large US railroads are capacity constrained and would rather spend their limited capital funds (after operating costs and stockholder dividends) on more or better track. Oh yes, and on PTC.

So while electric looks really good in many respects, I don't expect to see it happen on US railroads unless there is a high-frequency (probably commuter) passenger service on the line too. In that case the electrification will be justified based on passenger service needs, with the (usually small amount of) freight handled by diesel as now. It would probably take vastly higher diesel prices (perhaps north of $10/gallon in 2014 dollars), lack of fuel availability (unlikely, if willing to pay the price - biodiesel works as well as regular diesel, and can be produced from a variety of non-food-crop sources), or some environmental/energy edict (if it would even be enforceable) that prohibits use of local combustion power for rail systems, to make people even look at electrification seriously for the general railroad system.