In an experiment that's been going on for a few weeks, printing company Shunkosha has been deploying its strap covers on parts of the Tokyo subway to prove the effectiveness of Strappy.

Strappy is a boxy little plastic covering that sits over the strap. Many similar strap covers already exist on Tokyo trains, but they're for old-fashioned analog ads, and sometimes QR ads.

Strappy functions as a platform for marketing communications, when passengers touch a compatible smartphone to the Strappy cover, their browsers are directed to a URL with ads or other info.

You'd think it wouldn't work very well with weak phone signals in between stations, but carriers NTT DoCoMo, KDDI, and Softbank are currently installing antennae in tunnels to provide service throughout the subway, which carries more than 8 million people per day.

The Shunkosha trial has so far featured strap-sent ads from H.I.S., a major travel agency, on the Ginza and Marunouchi lines. It's scheduled to wrap up this month.

Strappy’s technology is simple, but is a taste of what’s coming down the road.