Although the technology is generally a novelty elsewhere, gadget-loving Japanese commuters have embraced the cards – plastic embedded with a tiny computer chip, permitting payments without the hassle of coins or making change.
Here, the challenge is not getting people to use them, but figuring how to expand the system for shopping, concert tickets and other electronic wallets.
Similar cards have been introduced or tested in Washington, D.C., France and Hong Kong, and by U.S. retail chains like Target and ExxonMobil.
But Tokyo commuters outdo them all: Some 5.6 million people have the green-and-silver Suica cards - about half the people who regularly ride on the train lines served by the card system. East Japan Railway Co. introduced the cards about a year ago.
"It's so convenient," says 18-year-old Yusuke Hirohama, a high school student, who uses Suica just about every day. "It's a breeze to use."
East Japan Railway decided to switch to Suica because the old ticket-reading machines and old passes were getting worn out, says spokesman Kazushi Masuya.
A play on the Japanese expression "sui-sui" (pronounced "sooh-eeh sooh-eeh"), which means "zip on by," Suica stands for "super urban intelligent card."
Unlike most commuter passes and cards found elsewhere, these smart cards work from up to four inches away.
Most Japanese don't bother to take their Suica out of their wallets, coat pockets or even bags as they zip on by. That can be critical in a crowded, uptight city like Tokyo where dozens of rushing commuters are moving up behind you.
If people try to pass through without paying, a turnstile flips out to block their path and an alarm goes off.
Suica's success among commuters could be a fluke.
Similar technologies that have been introduced in Japan have yet to reach the scale of Suica.
The commuter cards caught on quickly because millions were already familiar with the railway's more primitive card system, which used magnetic tape for commuter passes.
Another reason for Suica's success is the sheer flux of commuters in Tokyo, the long lines for tickets and the frenzied hurry that everyone seems to be in.
Sony Corp., which developed the Suica technology, also runs its own smart card service called Edy, short for "euro, dollar, yen." About 2,100 stores in Japan accept the 650,000 or so Edy cards in circulation.
The Edy cards, for instance, can be used for groceries at a convenience store or lunch at a restaurant. Instead of fumbling for cash, just place the card next to a display at the cash register.
But when Sony tried to sell tickets to a recent pop-music concert through Edy - letting people punch in their Edy numbers on a Web page and then flash their cards at the door - only about a dozen Edy-holders showed up.
Nonetheless, now that Suica has caught on, users are looking to do more without having to turn to other cards.
Reflecting common sentiment, Hirohama's only complaint is that Suica can't be used on the subways, which are run by different railway companies. And shopping with Suica would be nice, the student commuter mused.
The possibilities are enormous.
Tokyo train stations are brimming with newspaper stands, coffee shops, soda vending machines, drug stores, even restaurants and fancy shopping malls - mostly run or leased by Japan Railway or its subsidiaries.
But the company - a former government monopoly that takes pride in a stodgy image of public service rather than competitive marketing - says it has no immediate plans to expand Suica as an electronic wallet. It says it's still studying the options.
In June, Suica will hook up with Visa and JCB credit cards. But the card merely combines the Suica and credit card into one card.
Their functions aren't electronically linked, so you would still need to go to special machines to add money. The credit card part works like a regular credit card. You just don't have to carry two cards.
Sony senior manager Shusaku Maruko says the key to success is how widespread it can become.
"Is the neighborhood green grocer going to invest in this?" he said. "Or is he going to insist, `I just deal in cash'?"
By Yuri Kageyama