Before heading into the subways together each morning for work, police officer David Machin and his partner, Cyrus, wake up together in the Brooklyn apartment they share. Mr. Machin has had five partners in his 10 years on the police force, but Cyrus is the first partner to share his home.

Cyrus is also his first partner that is a dog.

Cyrus, a German Shepherd, is a founding member of what will become, over the next few months, the nation’s largest underground canine unit. As they complete an 18-week intensive training course, 27 German Shepherds will begin patrolling subway platforms, cars, mezzanines, and tracks with their handlers as part of new subway counterterrorism and crime-deterring initiatives.

The city has not had a separate canine unit trained specifically for subway patrol since the 1990s.

“The biggest issue is the deterrence,” the lieutenant in charge of the transit canine division, John Pappas, said. “I don’t know how many terrorist incidents the dogs have prevented because our enemies are watching what we do. The presence of a dog on the platform has an effect. People respect the dog.”

The German Shepherds, bred in Germany, Hungary, and Slovakia, are trained at Brooklyn’s Grand Army Plaza station by the police officers with whom they live and work. They arrive from Europe when they are about 15 to 18 months old, so they can grow up in their work environment. Mr. Pappas estimated that the dogs cost the police department about $5,000 apiece, including training.

Responsibility for naming the dogs is given to each handler.

As the dogs make their subway debuts, some New Yorkers say they are surprised to see them on subway cars.

“It’s fine if they’re on the trains, as long as they don’t come anywhere near me,” a subway rider on the downtown no. 6 train, Ava Taylor, said. “That thing looks like a wolf!” So far, Mr. Machin said he doesn’t miss having a human partner on the beat. “I love dogs, and I’ve had pets my whole life,” Mr. Machin said. “Cyrus keeps me company.”