These things are a choice though. The natural monopoly isn't the trains, it's the tracks. And it's not maintenance of the tracks, it's ownership of them.

So you have the government own the tracks, contract with a private company to maintain them. Then anyone can use the tracks, like anyone can use the roads. Private companies offer train service to the public. All they need is rolling stock and they can start selling tickets. You then get a market that looks like airlines, i.e. entering the market is a moderate investment (millions; buy rolling stock/planes) rather than needing billions to build the network itself. More popular routes get more suppliers, which turns into more frequent service. There is plenty of competition because rolling stock is mobile and can easily be reassigned according to customer demand.

Open access rail operators are not permitted to compete with franchisees (on the same routes/proposition) - there's a test that is applied before granting a license to ensure that the services will be "not primarily abstractive" - that is, that the operator will generate new revenue rather than simply taking away from the franchisee.

Is the slower train cheaper? If so, isn't that a competitive market choice?