It's true that politicians talking about shared values and ideals can turn simple private goods into nightmarish public ones (e.g. US medicine). Have politicians created dumb laws like "the conductor may not kick someone off the train even if they don't pay" or "long trips must cost the same as short ones"?

The train is merely an intermediate capital good used to provide train seats. It's private at the level of capital goods which is also the level at which it's paid for - only one railway can use a piece of rolling stock (rivalrous) and that railway can prevent other railways from using it (excludible). You might as well say that a house or a car is a public good, since they are merely capital goods that provide a stream of an underlying consumer good (space to live, road trips).

What are the positive externalities of a train? If anything, it's externalities seem to be negative - it creates pollution and noise. (It might be less negative than driving but that's a different claim.)