It would be different if, say, CenturyLink were blocking content across their transit infrastructure. That would be more like your railroad analogy.
There is a longish recent staff report on the Web of the Congress regarding behavior of Amazon, Apple, Google and Facebook (almost 500 pages) [0]. I actually read most of it. These platforms have already engaged in a lot of stuff that is anti-competitive. And the latest purge of Parler is, among other effects, also a suppression of a potential competitor.
Of course, the tech giants say that they weren't officially motivated by a desire to suppress a competitor, but they acted against potentially competing firms so many times already that one should be wary about their explanations.
[0] https://judiciary.house.gov/uploadedfiles/competition_in_dig...
It seems more like poor planning on their part: over-reliance on specific services of a single provider, and no disaster recovery plan for if this is made unavailable. Particularly considering the business they're in - publishing controversial content that gets its users banned on more mainstream social media sites. They would have been wise to have a backup plan for hosting.
On the other hand, if you were also passing reference to the app store duopoly (as the report mentions this), I agree with you on that. At least on Android a banned app can be fairly easily side-loaded; iOS users are entirely subject to Apple's whims on what their device is permitted to run.
If Amazon acted alone, I do not think that there would be an antitrust case. But given that Amazon acted together with the app store duopoly, in what appears to be a coordinated action with the same goal, I think the antitrust case applies. If two very dominant players make a pact with a third not-so-dominant player to squeeze somebody a bit more, it only makes their anti-competitive behavior harsher.
BTW, I believe that Parler really could have prepared better. It is obvious that they did not have a good plan B.
Mikhail Gorbachev used to say "Trust, but verify." I would like to see some verification in this case.