Government says you're not allowed to ban speech on your property. Tough shit, they say, you're a public forum and no one can avoid using you. Twitter is basically the railroad! But, yes, ok you can ban "hate speech," and presumably illegal speech, like repulsive pornography.
Who decides what the hate speech is? It's not longer the platforms, their owners, or their users. Users can't leave a platform if they dislike the hate speech policy, as every other platform is homogeneously bound by government regulation to allow and disallow the same forms of speech.
The government is now in the business, by virtue of requiring no private censorship, of defining what "acceptable" speech is. What if the political environment becomes even more fraught, and one side makes the argument that the core ideology of the other side is fundamentally 'hate speech?' Worst case, they're elected with enough of a majority to start banning such speech on all private companies' platform (hopefully at this point Americans are using platforms hosted in Europe). Best case, social media is a wasteland, with one side convinced by the norms of the time that the other literally shouldn't be allow to speak - it's hate speech! Which is defined and regulated by the government! Controlled by my party!
To the point of your analogy, speech in the form of sending an HTTP post request to a private website is not the same as traveling on a railroad. The forms of traveling on a railroad while behaving horribly are well defined and easy to regulate. Ditto the carriage of information over wires for the internet - that should be a common carrier. Running a social platform is not the same as carrying unknown packets of information.
When information so carried by a common carrier is composed to make complex political speech on privately-held platforms, which are themselves expensive pieces of property which exist to make a profit, the government should back off. The platform is itself an expression of free speech and free enterprise - it is not common property, and despite the over-puffed impression of Twitter lately, it is not, by any means, the only public forum.
To stretch your analogy, this is more like the government defining what sort of conversations can be allow among the passengers of the railway.