Metro is starting to sell unlimited seven-day rail passes on plastic fare cards instead of just paper this week, a boon to riders.

But it will be another few months before it may make sense for many riders to start buying them.

Within "a little less than a year," Metro plans to let riders use SmartBenefits to pay for most of its pass options, said Metro spokeswoman Morgan Dye. She did not have a specific time table but said riders could use the employer-given perk to pay for passes at some point in the first half of 2014.

Who should buy a short rail pass?
Metrorail riders who travel regularly during a seven-day period but take mid-length trips costing $3.50 or less per trip could benefit from the $35 pass. Here are some examples:
• A rider who travels daily between Fort Totten and Anacostia on the Green Line during peak hours normally would pay $3.40 per trip. If the person took at least 11 peak rail trips on that route in a week -- to and from work each weekday plus one extra peak-hour trip -- the pass would be financially worthwhile. Any off-peak trips would also be free.
• A rider traveling between Farragut North and Silver Spring on the Red Line would normally pay $3.55 during peak hours. With the unlimited short-trip pass, the rider would need to pay 5 cents to cover the difference. That extra fare used to be payable only as exact change at exit fare machines, but the plastic cards will allow it to be deducted automatically from the card balance. If the rider makes more than 10 trips, the pass still may be worthwhile, even paying the difference.
• A tourist visiting for a week who plans to take Metrorail repeatedly during the day, because any trips costing less than $3.50 will be free and the rider will need to pay only the difference for trips costing more.
The passes do have drawbacks, though. They do not work on buses and do not discount rail-bus transfers.

The change will make the passes even more attractive for regular commuters.

Currently, Metro has one-day unlimited passes costing $14, the seven-day short pass for $35 for unlimited trips costing $3.50 or less, the seven-day fast pass for $57.50 for unlimited travel and the 28-day pass for $230.

But the transit agency had been selling the $35 "short trip" pass only as paper cards until Monday. It will cease selling the paper version on Friday.

The short-trip rail passes have been the second-most-popular rail pass on the system, behind only the daily passes that are snapped up by tourists.

Yet even as paper fare cards, the short-trip passes have been doing much better in the last year since the July 1 fare hike, when riders saw across-the-board jumps in fares. Metro sold 13,114 in April, compared with 6,275 in 2012 and 6,670 in 2011.

Allowing the passes to be placed on reusable plastic fare cards that can be reloaded online or at any fare machine makes them all the more attractive.

But SmartBenefits will make them even more so. Some 270,000 local riders use the perk for up to $245 per month in transit fares, which can either be deducted from paychecks before taxes or given outright by employers as a benefit. The federal government pays the tab for many of its local workers -- who make up about 40 percent of the morning rush.