Question: What is Metro's most used mode of transportation?

Hints: It's not buses, which provide about 450,000 rides daily, or trains, with about 300,000.

Answer: Metro's hardest-working people movers are its 362 escalators, which give about 1.2 million rides on an average weekday.

When everything works, Metrorail patrons can travel everywhere without the exertion of a single step up or down. It is a remarkable technical feat, facilitating deep tunnels and making transit fans of people who either are unable or unwilling to make long ascents out of the ground.

But the cost has been staggering. Metro has spent about $80 million buying and installing escalators to date, or about 2 percent of total outlays for rail construction. Running and maintaining them consumes about $3 million more each year, about 2 percent of the rail operating budget.

Additional bills result from personal injury suits filed by patrons who take a tumble down the sharp metal steps. And escalators on the blink build ill will in a public that Metro has trained to see free rides up and down as a civic right.

When Metrorail was designed in the 1960s, the idea of stepless transit was nothing short of revolutionary in this country. It would be costly, planners knew, but nothing was too good for the future subways of the nation's capital, which visitors from around the world would ride and admire.

With a few changes, Metro has remained true to that philosophy. (Elevators were added on court order later to open the system to people in wheelchairs.)

Today, people typically use at least four escalators for every train they board. The smooth and near-silent flow of the grooved metal steps -- the standard speed is 90 feet a minute -- gives little evidence of the jungle of belts, pulleys, gears, chains and motors that lurks below and the machines' enormous demands for oiling, parts and mechanics' time.

"We get called SOB and everything else," said escalator mechanic Aubrey Thomasson, as he worked on a shut-down unit at Potomac Avenue station recently. "It angers people to have to pay the fee they pay to ride Metro and then have to walk an escalator."

George Bretz, Metro's escalator chief, argues that his machines don't deserve their reputation for constantly breaking down. Statistics show that, minute for minute, they perform between 97 and 98 percent of the time they're supposed to, Bretz says. But, he admits, it's the immobile ones people remember.

Metro's escalators are produced by Westinghouse, which also holds a $2.2-million-a-year contract to maintain them. Instead of the single large motor that other manufacturers use to drive long escalators, there is a small 10-horsepower motor every 20 feet, mounted just beneath the steps.

"Theoretically, we could build them to the moon," says Westinghouse's James Chambers, who directs a maintenance staff of 30 mechanics from a shop at Union Station. A major limitation, though, is the passengers themselves -- they grow restless on the long rides. The 139-second trip at Woodley Park-Zoo (Metro touts these escalators as as "the longest in the Free World") is considered to test the limits of patrons' patience.)

A typical escalator running empty 18 hours a day consumes about $750 a year in electricity. Carrying a full load of people up, the machine needs much more. But if it's going down, and five or more people are aboard, consumption is almost nothing, because people provide power like water turning mill wheel.

Each unit has a collection of switches that shut it down if a foot snags between steps and sides, or a hand gets drawn into the hole where the handrail disappears or a handrail snaps in two, or things are running too fast, or too slow. Designers tried to think of everything.

There is no single cause that knocks escalators out of service. Any one of thousands of finely engineered parts can go out of synch and cause ills like the handrail that falls behind the steps or stops altogether, the rough spot that throw patrons off balance, the metal shafts that snap and bring motion to a halt.

Like cars, some escalators are finicky, while others purr along for months without a hiccup. Maintenance crews know the up escalator at the Judiciary Square entrance by the old Pension Building as a lemon. (That is a particularly sensitive place because Metro board members routinely pass through on their way to Metro headquarters a half-block away.)

Surprisingly, the monster escalators like Woodley's are no more prone to breakdown than the short ones, Bretz says. The reason is that escalators tend to go wrong at the top or the bottom, where steps fold and flip over amidst many moving parts. Everything in between, regardless of how long it may be, is comparatively trouble-free.

Another surprise: rain and snow that fall directly on the outside units are not a major cause of service problems. The bronze plates and aluminium steps resist corrosion and water generally drains away harmlessly into spillways underneath.

The elements did, however, succeed in turning the Free World's longest escalators into its longest staircases last winter. Water pipes beneath Woodley Park's trio froze during the January cold snap and burst, coating the undersides with ice.

Negligence takes a toll as well. Recently, an unknown soul dropped a screw into one of Federal Triangle's mezzanine-to-platform units. The screw became wedged at the top and diligently stripped off a metal ridge on each and every step as it was driven by. Each of its 60 steps had to be removed and rebuilt. The cost: $8,000.

Perfectly good units, meanwhile, fall victim to vandalism. People drop bricks and newspaper vending machines onto steps and slash handrails. Metro has budgeted $93,000 this year for repairing their handiwork.

But Bretz cautions that idle escalators are not necessarily broken. Station attendants routinely turn them off to save wear and tear and electricity, or to foster the safe and efficient movement of people.

For instance, if a platform is filled to capacity, it makes sense to shut off the down escalator from the mezzanine. Leaving it on would feed people down too quickly and risk pushing someone onto the tracks.

As a safety measure, Metro has already slowed the escalators' original 120-foot-a-minute speed to 90 feet. Studies show accidents fall off dramatically as a result. (Some of the world's fastest escalators, U.S. transit officials say, are ones in Moscow, which zip along at 236 feet a minute.)

Despite such precautions, Metro is constantly fighting off injury suits. Not long ago, a man raced to an escalator at Farragut West, tripped and tumbled down it, according to Bretz. It was off. He later sued on the grounds that his expectation it would be running caused him to fall.

Bretz complains that lack of common sense is often to blame for accidents. A prankster hits an emergency stop switches for the thrill of watching a load of unsuspecting people pitch forward. A mother lets her little girl sit on the steps and her dress gets snagged.

"You've got to treat it with some respect," he says. "It's a rotating piece of machinery with sharp edges."

Despite the costs and headaches, Metro remains inseparably wedded to the escalator. Current plans call for $40 million more in capital spending on the machines before on the full 101-mile rail system is complete.

But several years ago, the board did make one concession to costs. It ruled that asking people to walk down a drop of 24 feet or less was not unreasonable. Starting with Clarendon and Court House, stations were built with stairs for short trips down. People can still use the elevators, though.

Each staircase means about $80,000 in capital savings. Maintenance costs go down too. The same thinking is being applied at Metro Center, where work crews are putting in stairs between the two track levels to supplement the crowded escalators.