on the bus, in the car etc. vs. on a bus, in a car etc

I realize a lot of questions have been asked about buses here. While the thing with the preposition usage was resolved for me a long time ago, the usage of the articles is still confusing me.

For example, which article would be more appropriate, if you were traveling in a vehicle and your phone rang and you wanted to avoid a conversation:

"I can't talk. I'm on a / the bus (in a / the car, on a / the train) right now. I'll call you back later."

My logic is:

In a situation when we're not talking about the specific bus/car etc. and just stating the fact that we're traveling in a vehicle, the indefinite article should be used.

So I would use "a". However, Google gives plenty of examples of both "a" and "the" used in similar situations. "The" seems to prevail, though.

Can you please explain the difference between "a" and "the" in these phrases?

It's an interesting one.

Of course, the generally refers to a specific case, and a/an to a general case, so why do we use the in these phrases, where we don't generally care about which car, train or bus it is?

With car we would take it to refer to the car that person is most likely to use. If I called my girlfriend and she said that, then the car would mean her car. If I worked somewhere where people drove cars as part of their jobs and I phone a colleague, the car would mean the car they drove for that job. In both cases it means the specific car one would expect them to be driving. A car would sound strange, because I'd assume they would be in the particular car they are normally in. Even if I phoned someone who I knew nothing about, they might say the car because they are used to thinking of their own car as the car.

With public transport there's two slightly different cases.

One of them is that if it was reasonable for me to know they were going to use a bus or train, then clearly they were only going to use one, so the bus or the train is that which they ended up using.

Another is that we tend to apply a degree of synecdoche to bus and train (and likewise tram, ferry, etc.) where the individual vehicle applies to the transport system as a whole. This is why we often use on rather than in with such vehicles; when we're on a train we're on the train-based transport network in a way that "on a car" doesn't compare to (though "on a motorway/highway" does). And likewise this degree of synecdoche makes a particular bus stand for the entire mode of transport and so a bus becomes the bus.

I am on the bus = I am on the bus you would expect me to be on (or the bus that is going to take me home / to you / to our meeting place / the cinema etc etc ad infinitum.

hi where are you now -> ANSWER I am on the bus (THE situation = a friend enquiring as to where their friend is who is late for meeting etc )

how long until you get here --> ANSWER I am in the car right now won't be long. (the implied car is either of these or maybe more variants, THE PERSONS OWN CAR ,A FRIENDS CAR, A HIRED CAR occasionally it may refer to a TAXI dependent upon the habitual speech of the person answering, "but IN THE TAXI is most common terminology"

what is taking you so long where are you ? --> ANSWER I am stuck on the motorway, horrendous traffic sorry mate --- plus more info is added here very often by the person answering

the bus or train --> we say on for public transport for a bus or train that is part of a system

the plane --> we say on for same reason but sometimes your hear IN THE Plane, because the speaker realises that to be on a plane implies a dangerous stunt act of actually being "on the planes fuselage ON TOP OF IF so they say in the plane to correct the logic and override convention.

IN A SUBMARINE --> the logic of the plane example becomes more pronounced here it is impossible to be on top of a submarine that is submerged (and submarines came to be in existence after the English language was already laid down by convention.

Houston calling commander of space shuttle 16, where are you ? -->ANSWER can in this scenario actually and truthfully be either of these. "I am on the shuttle now" " I am in the shuttle " "I'm under /below the shuttle" "I am orbiting the shuttle" "I Am above / on top of the shuttle " "I am outside the Shuttle" " plus there are more answers all possible" because space travel came after the English language was evolved, maybe in time there will be laid down via usage etc conventions for an astronaut describing where they are in relation to a space shuttle, In response to HOUSTON CALLING where are you ?

FINALLY BACK TO THE BUS

Hi joe its Bob just thought I would call up my old mate, for a quick get together tonight, can you come where are you? --> ANSWER I am travelling in the back of a bus right now hurtling down the Motorway, call me back in 2 hours we'll arrange a met up.

no theories just actually facts on the conventional usage of ON / IN a car bus etc.

THE bus is same as ON bus >> it implies I am travelling via bus -- > THE / A BUS that is going to take me home / to my wedding etc .

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P.S additional later info to consider

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Thank you for your assistance it is much appreciated. However there are still some unexplained flaws in my logic, that only make a little more sense, when we take into account that the word "BUS" originates from the older word "omnibus" And omnibus means "a collection of things related". You can google for more info. Thus originally there were horse drawn omnibuses consisting of coaches full of people going to the same place for the same purpose. A funeral cortege could potentially be described in older English as an Omnibus. It seems that over time it has become convention that English says ON for public transport where you BOARD the transport, board a bus, train, plane, ship, ferry …. In a car or taxi by convention we seem to prefer this way of expressing it. When the vehicle is being used for you own use only. You would use a car or taxi solely for yourself or yourselves to go solely to your own particular destination. Therefore in this situation you are not an OMNIBUS of people.