I doubt that story. It's hard for me to imagine how one would verify it in any case.
Notice that mass transportation generally uses "on". "On the bus", "on the train", "on the plane".
On the other hand, cars and small recreational airplanes would be "in". While small vehicles that are not enclosed are "on": "on the bike" "on the motorcycle", etc.
My intuition is that it has something to do with the notion of boarding or embarking. I would never say I boarded my car. But to board something is basically to "step onto" something. Again, it depends on how the event is conceptualized. There may be a historical explanation, but again I would be wary of them without substantial empirical support.
Having said that, I think the more likely historical explanation would be that "on" is used for mass transit by analogy with traveling by boat -- the first form of mass transportation.
The Old English word lang has suffix forms -ling and -long, (“with the direction, duration, or length”), similar to -lic (“with the body or form”) that survives as the very common -ly adverbial suffix form.
The -ling form, once used in words like hinderling (“in the backward direction”) has been supplanted by another usage, the nominal diminutive: darling, yearling. The -long form still survives, with all three senses, in words like headlong, nightlong, and footlong.
The suffix is adverbial in directional modifiers like headlong and arselong, adjectival in metric modifiers like footlong and nightlong. When combined with a quantifier to form an adjectival or adverbial phrase, the metrics compounds break into their component words:
- I ate a footlong sandwich. [adjective]
- This sandwich is one foot long. [adjectival phrase]
- This sandwich measures one foot long. [adverbial phrase]
- We attended a nightlong party. [adjective]
- We stayed at the party the whole night long. [adverbial phrase]
As these express spatial and temporal dimension rather than relationship, it's not clear whether these are adpositions or simply adjectives with an idiomatic phrase order. However, the close similarity to a hole one foot through or party the whole night through suggests that they may indeed be postpositions.
Best Answer
Some sources (see Nordquist's article, especially the reference to Allerton's work) do list ago as English's sole postposition, but there are actually quite a few other words that pattern the same way:
and the following words, and others, have senses where they follow this usage, although otherwise they are prepositions and/or adjectives:
It's interesting that these all seem to result in phrases that act as adverbs of temporal or spatial extent.