It's a century-old tradition that still charms tourists but may be nearing the end of the line.

As cable cars roll down the city's famed hills, they're braked to a halt on a huge turntable. Out pop the passengers, followed by the gripman and conductor, who spin the empty cars 180 degrees and send them back on their way.The task, performed scores of times each day, yields smiles from tourists - and strained backs and knees for the workers.

"People are getting injured," said Mayor Willie Brown, who recently endorsed a $30 million plan to motorize the turnarounds.

The mayor's plan is backed by the Transport Workers Unions, which represents cable-car operators, but not everyone is on board.

"It's part of the mystique, part of the romanticism of the cable cars, to see these guys actually physically turn them around, and we think that should be preserved, if possible," said Dee Dee Workman, director of San Francisco Beautiful.

The job used to be so easy that it could be done with one hand, and for years passengers and bystanders also helped out. But the cars have gotten older and tougher to turn, and liability issues put an end to passengers offering their muscle.

Some operators say the years of strain have taken their toll.