With 10s of thousands of miles of lines, it would be pretty monumental to electrify the entire US train system. Considering how efficient modern diesel trains are, That should be pretty low on our list of things to convert.
Nuclear trains might make sense with some of the newer reactors... but we know that'll never happen.
True although even in Europe where almost all mainlines are electrified, many freight trains are diesel-electric. That's because often the the plant loading docks etc. aren't electrified.
Most freight is carried by diesel trains. While it is possible to use electric engines, the ecconmics don't work out and so only a minority of freight is on electric trains.
There are diesel trains in Europe. They tend to be more on branch lines, though, with the main trunks being electric. (Electrifying every branch is not economic.)
In the US, it could still be done (and has), but the economics are not as compelling because the price of diesel is lower.
I'm told that the big railroads in the US have a plan to electrify their main line. It remains a plan because the costs are more high enough that it isn't worth it. If they decide fuel costs will go up by enough they will dust off the plan and electrify.
