Many other utilities tend to natural monopolies in similar ways such as telephone service, especially to rural areas, but railways are an extreme case.
Online some network effects can create natural monopolies because users get benefits from being on the same platform as all the other users. It’s a very different dynamic though and different platforms can cater to different communities and use cases with complementary value. Twitter and Facebook for eg. They don’t impose a cost on the public in the way railways do either.