There's a particular type of truck that drives around Japan in the colder months, selling something delicious too. Maybe not what you'd expect, though - they sell baked Japanese sweet potatoes!
I remember the first time I tried one of the potatoes from one of these yaki-imo trucks. Close to where we previously lived in Tokyo, there was a van that cruised the streets each evening, playing a tune with words I couldn't really make out (I didn't know much Japanese at all, at the time). I assumed it was someone trying to sell something, but I didn't know what. One day, on my husband's way home from work, he timed it right so that he could catch the Yaki-imo truck, and got us one each. Racing inside and handing me a brown paper bag, he exclaimed "you'll never guess who was outside! Yaki-imo man!"
The sweet potato was so good! It was perfectly soft and delicious, after having been slow cooked for hours. I don't think I expected that first experience to start a bit of a yaki-imo addiction! Any time we're out and about and hear a yaki-imo van, we basically chase them down to get our fix!
If you've never had a baked Japanese sweet potato, you're missing out! Some supermarkets here will actually sell them - they're usually in the produce section, sitting in a warmer on top of the roasting coals. I've even seen them being sold regularly at my local Don Quixote store! As for the actual trucks driving around, I'd say that for the most part I've seen them more commonly around countryside locations as opposed to Tokyo, but we did have one a few years ago that was in our Tokyo neighborhood on a nightly basis.
If you do ever get a chance and see a little Yaki-imo van driving around, playing their catchy tune, give one a try - they make a delicious (and healthy!) snack!