At least 55 metro lines in 37 cities around the world are fully automated, according to the International Association of Public Transport, an advocacy group that promotes public transit.

Many of these fully automated lines are closed systems, in contrast to Sound Transit which, at times, mixes with other vehicular traffic. The agency says it will eventually study automating or semi-automating parts of the Link system, but today, ST uses a hybrid system with operators and computers working together to operate the train. Here’s how it works: