The policy is rooted in several concerns, including potential legal liability; the higher cost of hiring better-trained, better-equipped guards or police officers; and a belief among transit officials that the tunnel was safe, Mr. Jacobson said. Last year, only two arrests for assault were made in the tunnel, he said.

Now, said Harold Taniguchi, the head of the County Transportation Department, which oversees the transit agency, “we are reviewing everything.” The county has temporarily assigned armed sheriff’s deputies to each of the five stations while it considers revising its private security contract. Until last week, it often had no more than two regular armed police officers patrolling the entire tunnel. Typically, at least two private guards are on duty at each station.

Image Private security guards were on duty last Thursday at the Downtown Seattle Transit Tunnel. Credit... Stuart Isett for The New York Times

The reluctance of the guards to intervene alarmed many people and has increased scrutiny of the transit agency. In interviews at the Westlake Station one night last week, many people said that the guards’ behavior, as much as the crime, was what stunned them most. Many said they thought they would have acted differently, regardless of company policy.

A spokesman for Olympic Security Services, the company that employs the guards, said that all four who appeared in the surveillance video had good employee records and that one had been reassigned since the beating.

“Under the county policy, they did what they were supposed to, ‘Observe and report,’ ” said the spokesman, Laird Harris. “The question is, could they have done more?”

Jeff Flint, the executive director of the National Association of Security Companies, of which Olympic Security Services is not a member, said that it was not unusual for contracts to prohibit private guards from intervening in crimes, largely to protect against liability and keep costs down, but that a major transportation hub seemed like an unusual place for such terms to be applied.