The Freedom Train: why a train?

I have been wondering why some references to a "Freedom's Gate" in culture have taken on the form of a Train. This is one of the things which cannot be an ancient symbol per se. So how did it enter Consciousness?

a) Why can't it be simply a train, and does it have to be a Stargate reference?
It depends on how this train interacts with the rest of its environment, and how it is framed. And in all these cases, it does act as a Symbol - being in the centre of attention, and a vehicle for certain feelings & emotions to do with a change in perspective, an escape from limitations, etc.
b) Maybe all of these people are just referencing each other?
It is possible, especially in this day and age of speedy information. But even so, the question remains "who was first?" and "what was behind THEIR specific way of framing the symbol?" - and why do we keep getting all these "Freedom" trains (which obviously do work separately, on each individual level where they are referenced)?

I think the answer may lie in American history - the abolishing of slavery, in particular. I was pointed to this answer by the story of the song Rock Island Line, about the train which smuggled slaves to freedom.
Likewise, Martin Luther King Jr. held his speech about using Faith to carve a tunnel in the mountain of despair.

The Symbol of the Train is not used just to escape one condition, but also to visit others: think of the Hogwarts express, for example, taking the students of Magic to another dimension where they are to be trained.

It is also a symbol of the Inevitable.

A variation of the Train would be the Underground or Subway (affectionately called Tube in London), which is even closer to symbolizing Stargates (which are supposedly tubes): it quickly connects different places in the city.