Among the compelling reasons to change subway cars while a train is in motion are a threatening situation, a malodorous neighbor, and the eternal hope of a rush-hour seat. Soon, however, riders may face a $75 fine for moving between cars.

It will be up to a police officer to decide whether a rider was justified in flouting the new prohibition, under a plan scheduled to be ratified tomorrow by the Metropolitan Transit Authority and take effect October 1.

Yesterday, a subcommittee of New York City Transit, the MTA agency that oversees the subways, voted unanimously in favor of the rule, despite reservations that police officers would be unable to enforce it uniformly and that, often, the doors to the next car offer riders a passageway to safety.

Transit officials said the new rule would strengthen an existing ban on riding between cars, a ban meant to deter “car surfers.” That ban, which likewise is accompanied by a $75 fine, is also intended to make it harder to sell goods or to panhandle on the subways, which are both illegal practices.

Broadening the previous rule, to cover movement between cars as well as riding between them, would also serve to diminish the MTA’s vulnerability to lawsuits, officials said. Even so, the MTA has had “favorable rulings” in three lawsuits brought against it since 2002 after riders were injured or killed while moving between cars, a transit spokesman, Paul Fleuranges, said yesterday.

The president of New York City Transit, Lawrence Reuter, said, however, that safety was the overriding concern.

In the past 10 years, transit officials have recorded at least 13 fatalities and 117 injuries of passengers riding between cars while the train was in motion, Mr. Reuter said.

“In order to try to eliminate these 117 injuries and 13 fatalities, we need to stop people from moving between cars,” he said.

Of the 41 people who died on the subway tracks in 2004, at least two died while riding between or on top of cars, according to figures from New York City Transit. Police said the number is as high as six.

The rule against riding between cars was originally proposed early in 2004, along with a highly contentious initiative to ban photography in the subways. Yesterday, transit officials voted in favor of permitting photography in the subway, though tripods and other equipment may not be used.

As for leaving one car for another, Mr. Reuter said: “People can still move – if there is a hot car or there’s a problem – at a station. It’s generally no more than two or three minutes between stations. Get out of the train and move to the next car. If there is any type of major problem, our people on the train, the motorman or the conductor, will authorize you to move, or a police officer will let people move from one car to another.”

The chief of the city’s transit police also said the new ban would not be enforced insensitively.

“If they’re moving between cars because of a foul odor … that’s reasonable. That’s not going to be summonsed,” the chief, Henry Cronin, said, giving one example. “Again, common sense has to prevail, and our officers use straight common sense all the time.”

Such subjectivity, though, worries critics who feel the rule will be applied arbitrarily. “If they have a rule, it needs to be easily understood by the public and easy to interpret by law enforcement officers, and this doesn’t pass that test,” a spokeswoman for the riders advocacy group the Straphangers Campaign, Neysa Pranger, said.

Even the chairman of the New York City Transit Committee, Barry Feinstein, conceded that he sometimes has moved between cars on the crowded shuttle between Grand Central Terminal and Times Square. Mr. Feinstein, the former city Teamsters leader, eventually voted in favor of the rule, acknowledging his own habit to be potentially unsafe, but first wondered aloud how the ban could possibly be enforced.

“In my experience I have heard the PBA scream bloody murder at a City Council hearing about ‘You’re not going to make a rule’ and ‘How the hell are we going to enforce that rule?'” Mr. Feinstein said, referring to the police union, the Patrolmen’s Benevolent Association. “I view this in much the same way.”

Mr. Feinstein asked that Mr. Reuter conduct a six-month study, beginning in October, to analyze whether the ban results in increased summonses and decreased injuries. Other critics took a broader view, saying the rule turned the MTA into an arm of what a former parks commissioner, Henry Stern, founder of New York Civic, called “the nanny state,” the excessive control by a public entity over individual welfare.

“Banning travel between cars would turn the subway car into a prison cell, and there’s no reason to subject people to that treatment,” Mr. Stern said.

Perhaps because of the uproar over the proposed photography ban, very little formal opposition to the between cars rule was registered with the MTA. At yesterday’s committee meeting, Mr. Stern was the sole member of the public to comment.

Riders who were interviewed in the subway system yesterday, however, acknowledged the danger of riding between cars but said the option to move from one to another was one they would rather not relinquish.

“I’ve been in a situation when someone was getting harassed by some kids. If I hadn’t been able to get out of the car, I wouldn’t have been able to help,” one rider, Shoshana Osofsky, said. Ms. Osofsky said she was able to move cars to alert the conductor.

“If you’re trapped in a car and if they’re doing stuff to you, it’s like being trapped in a burning building,” she said.

Design factors make the new ban moot in much of the subway system: For safety reasons, about half the trains on the subway, especially longer ones that travel routes with sharp curves, already lock the doors between cars.