As human beings, we like to categorize things — it makes it easier for us to understand the world. We do that in transit, too. But sometimes these categories actually make transit systems worse.

Every metro area map in Trains Buses People has a key like this one, listing modes of transit.. These categories are very commonly used in transit, and that’s because they’re useful. A green line, for example, is a streetcar, and a red line is heavy rail. They’re very different technologies. One shares lanes with cars, one has its own tunnel. One can carry 2000 people in a train; one carries 150. The experience of riding them, the kinds of trips they’re useful for, and their role in a transit network will be different.

But mode distinctions are fundamentally made on technology, and sometimes technology isn’t what matters most.

Consider the San Francisco Bay Area, and especially BART and Caltrain.