For many routes, planes/trains/coaches are substitutes, so it's meaningful to say that trains are not monopolies. How do we know? Because when train tickets go up in price, there's a measurable effect on the number of coach tickets bought.

Your example (household electricity vs. diesel generators) doesn't work in the same way. If domestic electricity prices in London were to go up by 25℅, the number of diesel generators and amount of diesel fuel sold would not budge. Because diesel isn't a reasonable substitute.

If anyone wants to know more, search the internet for 'cross elasticity of demand' and 'definition of substitute'.

That's like saying HBO has a monopoly because only they can broadcast game of thrones.

If a product must have a perfectly identical substitute to not be a monopoly, then more or less all companies have monopolies. I feel that position is untenable.

Public utilities are regulated, which require them to make their case in front of a judge for price changes.

Much of the Olympics wasn't live for the west coast even if you watched on TV.