Overheated riders may be relieved to know that Metro has plans to upgrade its cooling system, spending $34 million over the next five years to replace aging cooling towers, chillers and ducts.

But Metro officials warn that those investments might not make the stations any cooler.

The air cooling system was never intended to bring temperatures down very far, said Pat Porzilla, chief engineer at Metro. It was designed to provide "sensible cooling," offering some relief to riders on the hottest days by aiming to keep the temperature around 85 degrees, a number selected in the 1970s with energy conservation in mind.

As temperatures outside climbed to the 90s over the past few days, even dark underground Metro stations have not been spared from the heat.

An informal survey taken over the past two afternoons with a $31.99 indoor/outdoor digital thermometer from Radio Shack showed temperatures ranging from 84.7 degrees at a nearly empty Rosslyn platform to 91.6 degrees at Pentagon Station during the afternoon rush period.

Some transit systems, including New York's subway, do not have air conditioning in stations. Many Metro riders said any reduction in temperatures helps during heat spells, but others said they wish it could be much cooler.

In preparation for their daily commute on Metrorail, some people bring a book. Tourists might pack a pocket-size transit map. Martin Carter brings a bath towel.

"I call this the sweatshop. That's what it is," said the 46-year-old who commutes downtown from the Branch Avenue Station to his job at the U.S. Department of Agriculture. While mopping his face and neck with the white towel, he said waiting for the train can be the hottest part of his day.

At Smithsonian Station, where Carter was waiting, the temperature was 89.1 degrees at 5:30 p.m. on Monday, cooler than the day's 93-degree high at Reagan National Airport. But the only thing Carter was counting were the lines of sweat dripping down his back.

One stop away at L'Enfant Plaza, while Saheedah Brice, 23, waited for a train, beads of perspiration collected on her face and shoulders. She asked the question probably on many people's minds: "Is there any air conditioning down here?"

Metro spends $13,157 a day to circulate cool air into the cavernous stations. Lowering the temperatures in stations would be "cost prohibitive," Porzilla said. Cooling the Metro, he said, is like blasting the air conditioning in a car with the windows rolled down. The cold air coming out of vents has to compete with hot air flowing down the escalators through the open entryways and being pulled in from the tunnels by trains.

Trains also add heat through their brakes and propulsion systems, he said. The lights add heat, and so do the hundreds of thousands of red-faced, damp-haired riders who cram onto the platforms every day.

At Federal Triangle Station yesterday, a crowd gathered on the platform as one train unloaded its passengers after a door failed to close properly. The temperature ticked up to 86.7 degrees on an afternoon when the outside temperature reached 97.

"Oh mercy, when will the train get here?" said Bernadette Peters, 56, as she used her hand to fan herself.

Many riders bide their time in warm stations till they can step onto a cool train, which has a traditional air conditioning system. Stations use a more energy-efficient system, commonly used by larger buildings, which cycles cold water, rather than chilling air directly. But when the train finally arrived, instead of receiving the cool air blast she was expecting, Peters stepped onto a doubly crowded car, with stagnant air that registered at 90.1 degrees.

Not every rider was so unlucky.

Robin Watkins, 40, said that the 87.1 degree Gallery Place-Chinatown Station felt like 1,000 degrees Monday afternoon, but when the train came, the doors opened and she claimed a choice spot beneath an air conditioning vent.

As the temperature dropped to 79.5, she was all smiles.

"Ah," she said, "That's lovely."