[rahimnathwani]

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'.