Metro riders who transfer from bus to rail soon will receive a discount on their rail fare, much as rail-to-bus riders have received discounted bus fares for years.

Riders who move from bus to rail within a three-hour window will receive a 50-cent discount when they pay their fares with a SmarTrip card.

At the same time, the 90-cent discount rail riders receive when they transfer to buses will be reduced to 50 cents, and the transfer window will be extended from two hours three hours.

Bus-to-bus riders still will receive free transfers and also will have their transfer windows extended from two hours to three, according to the plan.

The new system will cost the transit agency about $2 million a year, officials said.

Metro’s board of directors approved the measure in 2004, but with the requirement that all area buses be outfitted with SmarTrip card readers before the program was implemented.

Prince George’s County installed the systems in October, the last jurisdiction to do so.

A board committee told Metro staff Thursday that the panel would like to see the new transfer discounts in place by Jan. 4, the same day that Metro is planning to eliminate paper transfer slips.

Metro officials estimate eliminating the paper transfers and their accompanying equipment will save the transit agency $5 million a year, but board members have expressed concern that some riders may be unaware that they need a $5 SmarTrip card to receive transfer benefits when the change takes place.

Metro has given social service agencies throughout the area 50,000 free SmarTrip cards to distribute to low-income residents, but 19,000 of the cards remain unclaimed.

About 25 percent of Metrobus trips were paid for with paper transfers in August, according to the most recent agency figures.