Consider the following exchange:
A: How are you getting to Seoul?
B: I'm taking the train.
Note B's use of the. Usually, we use the when the listener knows which one. So, why doesn't B say a?
This is just an hypothesis, but I think it may have something to do with operating according to a fixed schedule at a high frequency. Public transport does this. Because of that, one vehicle is as good as the next: we do not distinguish between one bus and another as physical vehicles, but only as entries in a schedule. As all buses are more or less interchangeable, I think our subconscious may be thinking of them as one bus, coming again and again, at least in the context of taking the bus (not when you say "I was almost run over by a bus", or "a bus was blocking traffic"). Or we may treat the bus line as a single phenomenon. The same applies to trains, subways, ferries, and monorails, although in certain situations a/an is used—there are always exceptions. But aeroplanes don't seem to operate according to such a fixed schedule and route. And taking a plane is just more of a happening.
As an alternative, it is possible that this the is a remnant of phrases like "I will take the 6 o'clock train to London", which was probably more common when trains ran less frequently, waited a little longer for delayed passengers, and when taking a train was generally more of an event. When indicating a that you were going to travel by train, you were generally picked up, and so you would more often say "I'm taking the x o' clock train". People could call the station to inform whether your train had arrived yet, when they were supposed to pick you up. They knew at what time the six-o'-clock from London arrived. Later, as train schedules became more frequent, and people were not always picked up from the station any more, mentioning the time of departure became less universal, but people had got used to saying the train, so they kept that while leaving out the time. I must say that I find this hypothesis less convincing, but it is a possibility, mainly because of the definiteness in modern "I'm taking the train".