At long last, service is set to begin on the Silver Line this month. But not everyone is cheering.

Although people from Leesburg to Tysons Corner may be thrilled to have Metrorail as an option for traveling to downtown Washington and beyond, the outlook is gloomier for Blue Line riders, particularly those who take Metro from Alexandria and South Arlington.

That’s because later this month, when Metro begins its “simulated service” exercise, incorporating the Silver Line into the larger Metro system, Blue Line riders will have longer waits between trains. Starting July 20, Blue Line trains will run every 12 minutes, about four minutes more than the current average.

Laura Larrimore takes the Blue Line from her home in Alexandria to her job in Foggy Bottom, where she specializes in Web development. In April, she launched a “Save the Blue Line” Web site to raise awareness of service issues on that line. In June, she started tweeting her concerns and those of other riders.

Metro officials say it’s a capacity issue.

“We’ve known since the Silver Line was planned that to accommodate the new rail line, there would have to be offsets,” Metro spokesman Dan Stessel said.

There are only two ways across the Potomac River for Metro trains traveling between Virginia and the District. Yellow Line trains go over the Yellow Line bridge; Blue and Orange line trains move through a tunnel below Rosslyn in Arlington. The tunnel can accommodate 26 trains an hour. The start of Silver Line service means Metro has to open up some slots in the tunnel.

Larrimore acknowledged that Metro officials have to make hard choices to accommodate the Silver Line, but she wants to make sure that the concerns of Blue Line riders aren’t being ignored.

“Blue Line riders would accept some cuts, because we understand we need to share the tunnel, but these seem way more severe then they need to be,” she said.

Larrimore has been using hard data to make her case. By her calculations, if unlucky commuters always had to wait the full 12 minutes, over the course of a year, some Blue Line riders would spend up to an additional 15 hours waiting for trains to arrive. That’s the equivalent of two vacation days, Larrimore said.

“If my boss came and said he was going to take away two of my vacation days, I’d be really upset,” she said. “And that’s what Metro is doing.”

Metro officials said they understand that Blue Line riders are frustrated. And they are aware of complaints that Blue Line trains have become more crowded since the launch of Rush Plus — a reorganization of rush-hour service — in 2012. Recently, they announced that they are converting two or three six-car trains into eight-car trains on the Blue Line during rush hour, which could accommodate about 200 more passengers per train.

And they point out that not all Blue Line riders will have to endure longer waits. Some riders, particularly those going from Rosslyn to Largo Town Center, will have the option of hopping on a Silver Line train to get to their destinations.

Metro does have a more permanent solution in mind. As part of its multibillion-dollar Momentum campaign, Metro hopes to add a second station at Rosslyn. But even transit agency officials concede that’s pretty far down the line.

For now, Metro is encouraging Blue Line riders to try other ways to get where they need to go, which they’ve outlined in a brochure, “Blue Line — Consider Your Options,” that is being handed out as part of the Silver Line promotional campaign.

Inside, there isn’t a lot of good news for customers who prefer a one-seat ride, but we’ll try to sum up some of the options.

Blue Line — As Metro officials discovered when they launched Rush Plus service in 2012, some people prefer a one-seat ride. They don’t want to switch trains even if switching could get them to the destination more quickly. That attitude may be tested now that Blue Line trains will be running every 12 minutes.

Depending on the destination, people traveling from Alexandria and South Arlington into the District may be better off taking a Yellow Line train and switching at L’Enfant Plaza, rather than waiting for a Blue Line train. Metro offered this comparison: A trip from the Pentagon station to McPherson Square on a Blue Line train, with no wait time, will take about 11 minutes. Taking a Yellow Line train to L’Enfant Plaza and switching to a Blue, Orange or Silver Line train to McPherson Square will take only three minutes more. So, if the next Blue Line train is more than three minutes away, you might consider hopping on a Yellow one.

But if you’re headed to Blue Line stops farther west, it might be a tougher call. Larrimore said she tried taking the Yellow Line and transferring at L’Enfant Plaza to get to Foggy Bottom but found that it took far too much time. She was better off waiting for a Blue Line train. This option would be especially onerous to the thousands of commuters a day traveling to the office towers of Rosslyn from Pentagon City and points south in Alexandria and Fairfax County.

In a bit of good news for downtown commuters using the Van Dorn Street and Franconia-Springfield stations, Metro will be continuing Rush Plus service on the Yellow Line.

Orange Line — Once Silver Line service begins, there will be six fewer inbound Orange Line trains from Vienna during rush hour, and the time between those New Carrollton-bound departures will increase to six minutes.

Depending on their destinations, riders who board at East Falls Church will have the option of taking a Silver Line train to get downtown. Rush Plus service will be discontinued on the Orange Line, so if you’re headed to Largo Town Center, you’ll need to transfer to a Blue or Silver line train.

Silver Line — Silver Line trains will run every six minutes between Wiehle-Reston East and Largo Town Center during rush hour. During non-rush periods, the trains will run every 12 to 20 minutes.

About changes in service to Dulles International Airport: The Silver Line will get you closer to the airport but not quite there this go-round. For that, you’ll have to wait until 2018, when the second phase of the Silver Line is scheduled to open. For now, here are some bus options:

● Metrobus Route 5A will take you from L’Enfant Plaza to Dulles Airport, with stops at the Rosslyn station and the Herndon-Monroe Park & Ride lot. Monday through Friday, the bus will depart every 30 to 40 minutes. On Saturday and Sunday, it will depart every hour. The regular fare will be $7, which can be paid with cash or a SmarTrip card. Seniors and people with disabilities will pay $3.50. Check wmata.com/fares for additional information.

● Fairfax Connector 981 and 983 will offer service seven days a week from the Wiehle-Reston East station to Dulles Airport and will stop at Reston Town Center and the Herndon-Monroe Park & Ride lot. Route 983 will also include a stop at the Udvar-Hazy Center. Route 981 will have departures every 20 minutes; Route 981, every 40 minutes. Details are at fairfaxconnector.com.

● The Silver Line Express will offer nonstop service from the Wiehle-Reston East station to Dulles Airport every 15 minutes during peak hours and every 20 minutes at other times. The fare will be $5. Check washfly.com for up-to-date information.

In the meantime, if you have other Silver Line service questions, you can check out Dr. Gridlock’s weekly chat at noon on Monday.

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