When commuter trains get stuck in their tracks in both Maryland and Virginia, they turn to Metro to help carry their load. MARC riders can hop onto either leg of the Red Line, plus the Green or Orange lines depending on which train they take. Virginia Railway Express commuters can switch to the Blue and Yellow lines.

But no money changes hands during those commuter train failures. Metro honors the commuter train tickets, and allows the riders to be waved through the gate.

"When Metro honors MARC tickets it is considered transit courtesy," explained MARC spokesman David Clark.

It's not clear how many commuters have been allowed to ride Metro for free under these circumstances. No one appears to keep a reliable count, according to VRE and MARC. "We don't call upon them often," VRE spokesman Mark Roeber noted.

But VRE has relied on the Washington Metropolitan Area Transit Authority to honor its tickets at least six times in the past two months. In the past two weeks alone, MARC has done so on four separate days.

The relationship means that workers can count on getting to their jobs and their homes if the commuter trains fail, making them less likely to drive in a car, thereby reducing the number of vehicles clogging local roads. But it also means that Metrorail riders are subsidizing MARC and VRE riders each time.

Clark said the arrangement makes sense because the state of Maryland subsidizes Metro's operating expenses. "We are partners in a sense," he said.

Local jurisdictions paid a total of $572 million to subsidize Metro's current $1.5 billion systemwide operating budget, with Maryland's share just over a third of that amount. But Metrorail riders' fares cover 72 percent of every train ride, according to Metro budget projections, so their fares carry the bulk of the extra cost.

Metro does occasionally rely on the commuter trains to help out in the other direction, but far less frequently.

Roeber said he could remember two times in the past 10 years when Metro asked VRE to shuttle some of its riders. "We'd clearly pick up the ball if we needed to," Roeber said.

During President Obama's inauguration, VRE also let riders board at L'Enfant Plaza to relieve the boiling point pressure of crowds, though Metro never asked for the help, he said. "It seemed to be both the logical and the right thing to do," he said.