This is as far as I got on layman speculation. Different regions of a country presumably have different power plants feeding them, so either:
* There’s a changeover at regional boundaries where the train coasts under a short gap with no overhead line, so every section of a line is on a different power source (meaning load changes depending on how many trains are in the region);
* Each line is treated as one discrete circuit isolated at junctions, with one or multiple power plants feeding it (load is more stable but needs to be balanced between the plants);
* Every line is connected to every other line in one national or international grid, meaning load has to be very carefully managed both on the trains’ power units, and at the links to the power plants, and presumably lines have to be able to be isolated in the event of some freak accident to avoid it bringing down every train nationwide.