Mit dem Bus / mit einem Bus

Question

I've noticed that it is far more common to say "Ich komme mit dem Bus" than to say "Ich komme mit einem Bus". So I wondered:

• is it ok at all to say "mit einem Bus"?
• if it's ok, does it have a completely different meaning?

Since one normally comes with a general unknown bus, not his father's bus, say, I would expect that one will come with "einem" bus.

Answer 1

mit dem Bus means "by bus", mit einem Bus means "in a bus". Mit dem Bus/Zug/Auto/Fahrrad is just the fixed expression meaning "by bus/train/car/bicycle".

Here and in other places, dem is a definite article, but doesn't specify a particular bus, but rather buses in general. On the other hand in einem Bus would mean "in a particular, but undisclosed, bus".

I think the same happens in English:

I'm in the car at the moment. Can I call you back?
Ich bin im Moment im Auto. Kann ich dich zurückrufen?

I'm in a car at the moment.
Ich bin im Moment in einem Auto.

The latter sounds like you are in a particular, but unspecified, car.