The biggest, fastest-growing cities in the world all struggle with public transit. But necessity is the mother of invention, even when it comes to moving vast numbers of people through a 21st-century city–and many of these cities are either experimenting with new transit ideas or have pioneered transformative technology that’s now being adopted worldwide. Here are a few examples.

Rethinking Traffic Design

Traffic of any kind is a game of margins that are so small, most of us don’t even notice they’re there. For example, most of assume that the fastest and best way to deal with escalator traffic is to keep the left side open for people walking.

In fact, several cities are challenging that (and other) notions about congestion in subways. The Guardian‘s Archie Bland wrote last week that the London Underground is experimenting with a new idea in traffic flow, borrowed from cities like Hong Kong and Tokyo, both of which have actively promoted the idea of asking riders to simply stand on both sides of the escalator to allow more people to move through the station at once.

In all three cities, testing such a novel idea took serious effort; habit is a difficult thing to break. “That meant teams of staff standing at the bottom of the escalators with loudhailers, asking commuters, as cheerfully as possible, if they would mind standing on both sides,” Bland writes of London’s efforts. “It even meant asking amenable couples to hold hands across the escalator, the better to thwart those who wished to slalom through the line.”

Yet the effort may pay off. In super-dense cities where subway systems are already pushed to the breaking point with new ridership, this seemingly innocuous piece of etiquette—which saves only a few inches per rider—could end up drastically reducing congestion when you extrapolate across millions of riders.

UX Isn’t Just For Tech

Commuting in fast-growing cities often means extraordinarily long rides. Some cities are experimenting with ways to improve that experience with entertainment that goes well beyond your internal critique of the ads on your train.

In 2015, the company that operates one of São Paulo’s metro lines, Via Quatro, teamed up with a publisher called L&PM Editores to create a program called Ticket Books. Each paperback contains an RFID chip loaded with 10 trips on the subway–readers could simply wave their tome over the chip reader at the subway’s entrance to ride, and a website for the project lets you add more credit to your book according to PSFK (as well as select a new read based on where you’re heading).