Good-bye to seats at rush hour and cars without a standing load. Metrorail is putting the "mass" back into mass transit.

At stations around the 39-mile subway system, there is evidence of the new order.

Car doors spring open at McPherson Square during the evening rush to reveal a barrier of shoulders, faces, briefcases and fur hats. Some waiting passengers fight their way on board. Others hesitate, wondering if the next train will be any better. The doors slide shut, catching on a purse.

For other passengers, the complaint is no seat on the train that always had one before shorter trains began running Jan. 3. Commuting by subway, formerly a chance to read the paper, is now a chore.

Take administrative assistant Sadie Bone, who rides the Red Line daily from Brookland-CUA to Farragut North. Even at 8:30 a.m. she used to travel sitting down. Now, she says, "the only time when I get a seat is at Farragut, when I'm getting ready to get off."

Metro officials shed few tears over stories like Bone's. A guaranteed seat at rush hour, they feel, is an absurd luxury. Trains are supposed to be jammed. "You're not running a cost-efficient system unless they're crowded," says scheduling official Millard Seay, who spent yesterday morning watching the crowds at the Pentagon station.

Metro lauds the approach of passengers like Col. Tom Fields, a regular on the Pentagon-Farragut West run. "If you expect to sit, I guess you've got a problem," he said yesterday while waiting for a train. "But as long as I can get on and get where I'm going, I'm basically satisfied."

The new order could bring changes in commuter tactics. Even before the switch, some Pentagon passengers were already beating the crowds by first riding toward National Airport, then changing to a lightly loaded inbound train. When the train passed through the Pentagon station, they were already settled in seats, with newspapers raised.

But some habits seem too deeply ingrained to change. Commuters form strong preferences on which car they ride in, viewing it as a personal victory of sorts to select one that stops right by their exit escalator and lets them beat fellow passengers to the top.

Habits like these may help explain why certain cars in a given train are consistently loaded to capacity, while others have plenty of standing room day after day, and sometimes even empty seats.

Under the new plan, eight-car trains have vanished from the Red Line, and are replaced by six-car trains. The New Carrollton-National Airport run, formerly all six-car trains, now has some four-car trains. Ballston-Addison Road has more four-car trains than before.

The goal is to build up the reserve fleet and to free cars to open new Yellow Line track, possibly as early as April. Some of the spare cars have been coupled together into three special "gap" trains that are stationed around the system and sent into service whenever a breakdown causes a gap in the rhythm of trains.

Formerly, a breakdown disrupted service for 15 minutes. With the gap train, the break is more likely to be held to 7 or 8 minutes.