Comment by iggldiggl

Another crucial difference is that energy supply (as in selling energy to end users – physically providing electric energy to the grid on the other hand does require sufficient physical transport capacity in some ways) and internet access are much more virtual things – the mere existence of an additional company offering those services doesn't directly congest the infrastructure as such.

Trains on the other hand are decidedly physical things that take up a significant amount of space on the infrastructure, and they do so as soon you start offering the service, no matter whether people actually use it or not. This means that railway networks can only support a very limited amount of competing companies before you start running out of capacity to run additional trains, and it's especially easy to run out of capacity when you're talking about mixed-traffic railways where fast long-distance services intermingle with slower regional and/or freight services.

And as soon as you run out of capacity, train operating companies have to start battling each other for train paths instead of passengers, a situation that can have completely different incentives which aren't necessarily best aligned with passengers' actual interests.

Another difference is that competing trains obviously cannot run at exactly the same time, which again makes competition less efficient because trains running at differing times cannot be perfect substitutes for each other, which becomes relevant once you add passengers' external schedule-constraints (having to arrive in time for work or whatever appointment they might have and maybe cannot really influence) into the mix. (Long-distance leisure travel is probably less affected by that, because people are regularly willing to flex their schedules for that, but other kinds of traffic aren't as flexible.)

This effect then only gets magnified further once connections come into play, because with different trains along the same route always having to be separated by at least a few minutes, it's impossible to offer equally attractive connections between e.g. a branch line (which might run only hourly or half-hourly at best) and all the various hypothetical competing services on the main line.