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

Question

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?

Answer 1

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.

Answer 2

'A' generally refers to any type of car/bus, etc, whereas 'the' refers to the specific bus/train that we use regularly and may be on at the time of this call coming. People do use both, but 'the' is the more common one.