If your cellphone seems to have better service when you’re riding Washington’s subway, transit officials say it isn’t your imagination.

A consortium of four nationwide carriers — Verizon, AT&T, Sprint and T-Mobile — is working “every night and throughout the weekends” to install fiber and equipment, Metro’s chief spokesman Dan Stessel said.

“On any given night, there may be as many as six or seven separate job sites on the system,” Stessel said.

Metro had a long-standing contract with Verizon for service in the system, but the transit authority was required to expand the service as part of a deal with Congress in 2008 for federal funding for capital improvements. The agreement required that wireless service be available throughout the Metro system by Oct. 16, 2012.

Stessel said Metro is “working toward that goal.”

In the region’s Metro system, the top 20 busiest stations are now online with the four carriers, Metro said.

Those stations are Ballston, Bethesda, Columbia Heights, Crystal City, Dupont Circle, Farragut North, Farragut West, Federal Triangle, Foggy Bottom, Friendship Heights, Gallery Place, Judiciary Square, L’Enfant Plaza, McPherson Square, Metro Center, Pentagon, Pentagon City, Rosslyn, Smithsonian and Union Station.

The work at Metro’s 47 underground stations is about 75 percent complete, Stessel said.

Work is also being done to install fiber-optic lines at the 39 aboveground stations in the rail system. That work is 50 percent complete, Stessel said.

“We want to provide customers with the ability to use cellphones and wireless devices throughout the Metro rail system using the carrier of their choice,” Stessel said. “This is important not only for customer convenience but for customer safety.”

Metro has been working to improve the coverage of cellphones in its system for several years. But it has hit snags because it hasn’t had enough safety escorts to accompany crews who are installing the equipment.

Stessel said the carriers’ employees need a top-level safety escort to accompany them when they work on the tracks, but only a limited number of Metro workers have that training. Metro workers must have at least one year of track experience before they can provide safety supervision to others, he said.

With the addition of more carriers having fiber-optic equipment in the rail system, riders will be able to send text messages, surf the Web and make calls, Stessel said.

Metro officials have said that the deal with the four carriers is expected to generate a minimum of about $25 million during the initial 15-year term of the contract. The agreements would generate an additional $27 million during five two-year renewal options, Metro said.

The work on Metro’s cellphone coverage comes as New York City’s subway system is for the first time expected to begin limited underground service starting next week. T-Mobile and AT&T customers will have access to service at four stations in the Chelsea neighborhood of Manhattan, according to an official for the New York City subway system.