How will the Orange, Blue and Silver lines squeeze through crowded Rosslyn Station? Careful choreography. (Alberto Cuadra/The Washington Post)

By this time next summer, after Metro’s soon-to-open Silver Line has been operating for a year, Washington’s subway will be handling 780,000-plus passenger-trips each weekday, or 4 to 5 percent more than the current load of about 750,000 boardings per weekday, the transit authority predicts.

Maybe that doesn’t seem like a major increase. But James J. Hughes, the Metro official in charge of planning for the thousands of riders that the Silver Line will add to the rail network, calls it “a shock to the system.”

“It’s a big, big jump,” says Hughes, whose job title is director of intermodal strategic planning. “And people will notice it.”

Some people will notice it more than others — especially Blue Line riders boarding at stations south of Rosslyn during peak travel periods. Their waits for trains will be longer starting Sunday, when Metro begins a week of “simulated service” on the Silver Line, and will continue from July 26, when the new line is scheduled to open for passengers.

“We start with the existing schedule and we tweak it and tweak it as best we can,” says Hughes. “But putting in a new line is a very big deal. It’s not something we do often.”

Part of the problem is that the Silver Line runs from a new station at Wiehle Avenue in Reston, through four new stations in Tysons Corner, then joins the Orange Line at East Falls Church and shares tracks from there to Washington. Orange and Silver line trains will flow into the Rosslyn station, on the west bank of the Potomac River, along with Blue Line trains traveling north to Rosslyn from stations in south Arlington County and Alexandria.

But the tunnel under the river, between Rosslyn and Washington, can safely accommodate only 26 trains per hour in each direction, Metro says. Currently, 19 inbound Orange trains and seven inbound Blue trains use the tunnel each hour during peak periods. With Silver Line trains soon to join the mix, something will have to give.

The solution that Metro’s planners have come up with, Hughes says, is to move 10 inbound Silver trains, 11 inbound Orange trains and five inbound Blue trains through the tunnel each hour during peak times. During the morning rush, that will mean two fewer trains per hour for Blue Line commuters heading into Washington from stations south of Rosslyn.

In transit parlance, the time between trains is known as “a headway.” Under the present peak schedule, south of Rosslyn, the hourly Blue Line headways are six minutes, 12 minutes, six minutes, 12 minutes, six minutes, 12 minutes and six minutes — for a total of seven trains every 60 minutes. Metro plans to switch to entirely 12-minute headways in that stretch of the Blue Line, reducing the number of trains to five per hour.

This won’t be the first time in recent years that there’s been a service reduction for Blue Line riders south of Rosslyn.

Two summers ago, those commuters were seeing 10 trains per hour during peak periods. Then Metro scaled back that number to the current seven per hour. Of the 10 trains per hour that used to leave the Blue Line’s Franconia-Springfield station at peak times, three were redesignated as Yellow Line trains. Like other Yellow trains — those headed north from Alexandria — the redesignated trains cross the river on the Yellow Line bridge, south of Rosslyn.

That subtraction of Blue trains and addition of Yellow trains was dubbed “Rush Plus” service.

“The reason for starting Rush Plus,” says Metro spokesman Dan Stessel, “was that we didn’t want to shock riders with this reduced realignment of service all at once” by suddenly cutting the number of trains from 10 to five with the arrival of the Silver Line.

Reducing the number from 10 to seven in the summer of 2010 “was really an incremental step to get folks used to this realignment” before the Silver Line began operating, Stessel says. “More Yellow, less Blue. Maximize the space on the river bridge. There’s space available on the bridge. But there’s no more room in the tunnel.”

In recent weeks, Metro has been promoting an alternative commuting route for inbound, peak-period Blue Line riders who board south of Arlington Cemetery. If the next Blue Line train is 12 minutes away, Metro says, riders should instead consider taking the next-arriving Yellow Line train, which will cross the bridge to the L’Enfant Plaza station.

At L’Enfant Plaza, riders can switch to any other subway line except the Red Line and continue to their destination. In many instances, Metro says, the trip will take less or the same amount of time as it would if the rider waited for a Blue Line train at the departure station.

Just as the Silver Line’s arrival will mean reduced peak-period service on the Blue Line south of Rosslyn, it will mean a reduction of trains for Orange Line commuters boarding at the three stations west of East Falls Church: West Falls Church, Dunn Loring and Vienna. The number of trains leaving Vienna each hour during the morning rush will decrease from 19 to 11 because of the Silver Line, Metro says.

Other current Orange Line riders headed into Washington — from the East Falls Church, Ballston, Virginia Square, Clarendon and Courthouse stations — will see more trains each hour during the peak period. Those riders will be able to ride either Orange or Silver line trains.

But they’ll have to pay attention to which trains they board. From Rosslyn, the Orange, Silver and Blue lines will share tracks through downtown D.C. But from Stadium-Armory, the point where the Orange and Blue now diverge, the Silver Line will share tracks with the Blue Line to Largo Town Center, while the Orange Line continues to New Carrollton.

The changes will take effect with the start of simulated service Sunday, a week before Silver Line trains begin carrying passengers on the 11.4 miles of new tracks from Wiehle Avenue in Reston to East Falls Church.

Metro says that during simulated service, Silver Line trains will run almost as they will during full service. But as they head toward the city, they will not pick up passengers at Wiehle Avenue or any of the four Tysons stations — Spring Hill, Greensboro, Tysons Corner or McLean.

And as they’re headed away from the city during simulated service, the Silver Line trains will not carry passengers beyond East Falls Church, although they will stop at each of the new stations as part of the simulated service.

The service reductions are directly related to the tunnel under the Potomac from Rosslyn — a bottleneck that can handle only so much train traffic.

“From time to time, there’s been discussion” about building another bridge or tunnel, Hughes says, but the idea has never moved beyond the chit-chat stage. As the Washington area’s population and Metro’s ridership continue to grow, “somewhere in the near future, we’re going to run out of capacity,” Hughes says.

Referring to the possibility of adding another route across the river, he says: “We’re telling people it’s time start thinking seriously about it, because it takes so long to accomplish. It’s time to start reaching a consensus about what we’re going to do.”