As if I didn't already have enough electronics in my life, I started carrying around a computer in my shirt pocket a few weeks ago. Fortunately, it's fairly unobtrusive, only a little larger and heavier than a few business cards. It doesn't even look like a computer, unless you hold it against a light to see the tiny rectangular chip in one corner.

This gadget is a SmarTrip card from Metro, the newest "smart-card" venture in the United States. Smart cards, which embed a tiny computer chip to handle storage, processing and communication, have been touted as the next big little thing for over a decade. They're supposed to replace every other card -- ID, ATM, credit -- in your wallet, and they'll even connect to computers, enabling you to withdraw "cash" without leaving home.

But reality hasn't come close to the pitch here; previous trials of smart cards in Atlanta and Manhattan flopped. Why? As the Gartner Group noted in a report this May, "The U.S. payment system works. The current combination of cash, checks, debit cards and credit cards meets nearly all consumer payment needs." Merchants, meanwhile, have been less than anxious to install a new kind of cash register without any customer base.

Metro aims to beat the odds with SmarTrip. "We provide the critical mass," said Peter Benjamin, Metro's chief financial officer. "At any given time, something on the order of a million to a million-and-a-half people have our fare media in their pockets." About 13,300 SmarTrip cards have been sold so far.

For now, SmarTrip works only on Metro trains and parking lots -- forget using it to buy your morning coffee. You'll also have to shell out $5 for the card itself, and most stations lack a full set of SmarTrip turnstiles. But it has definite advantages:

* You only have to touch a card to a SmarTrip "target," eliminating the wait for a turnstile to slurp in and eject a farecard;

*If your card is tapped out, the system loans you one ride, which you pay back when you next put money on the card;

* This thin plastic plate doesn't crumple like a paper farecard, giving it much better jeans-pocket compatibility;

* If you register your card at Metro's Web site , you can have a lost or stolen card, including the remaining fare value, replaced for free.

In the next year, Metro plans to put SmarTrip and farecard readers in buses (finally!) and hopes to expand SmarTrip coverage to other transit systems, such as Dash and Ride-On buses, Virginia Railway Express and MARC trains and area toll roads. More importantly, Metro and First Union will offer a combined transit-banking card this summer -- a first step toward that promised smart-card future we may still not reach anytime soon.

SmarTrip, sold at Metro Center and Pentagon stations, transit stores (Ballston, Crystal City, Rosslyn, Silver Spring and White Flint) and at http://www.wmata.com.