This picture shows a double stack container under electric catenary, so it's possible. The cost would be high for the length of tank involved. Electrification's non-green advantage is faster acceleration, which is easy to justify on busy passenger lines as you get faster journeys and potentially more trains, but not important for freight.

http://testplant.blogspot.dk/2013/09/electrification-clearan...

If you think longer-term, electric trains are realistic. If batteries are cheaper than Diesel, you could just put 2x20ft container at the front with batteries. They can be swapped en-route and used as battery storage when not in use. As far as I know, most newer freight trains are diesel-electric anyway so not sure if you couldn't just feed the electric engine directly from a battery without buying new engines.

Currently not feasible because of high battery prices but realistic in 1-2 decades. And losses are lower than for electrified rail.

Busy routes are still better electrified but the huge US freight rail network would likely be cheaper to convert that way.

Could they use a third rail system? https://en.wikipedia.org/wiki/Third_rail

Not really. Freight lines are usually built at-grade and occasionally intersect city streets. Caltrain tracks included. Some of the Caltrain stations doesn't even have elevated platforms.

It would be... unpopular... to have an electrified third rail, at ground level, that you have to step directly over at a station or running directly through a city street. Somebody would surely trip on it or it could get touched by a little kid.

Third rail systems are usually only used when the tracks are physically inaccessible to passengers or other pedestrians.

Plenty of third rail in the south of England, they just stop the rail before level crossings and start it again afterwards.