You certainly can run freight trains off of electrified tracks. E.g. the iron ore trains in Scandinavia, which go up to 8500 tons, using a pair of locomotives that together output 10 MW sv.wikipedia.org/wiki/Malmbananen.wikipedia.org/wiki/Iore
The US is running longer and longer trains as you have observed (up to a max of 14,000 tons), but that is only because they keep fucking over their staff so they quit, so they have to do that out of necessity due to a lack of drivers. It’s far more dangerous as it increases the risk of derailment, it means the trains can’t fit in sidings (which screws over scheduling as trains can’t pass each other) and ought to be stopped.