THE SMELL of burning rubber was overwhelming in the ancient Silverliner II railcars as SEPTA's crowded Paoli/Thorndale local left Suburban Station during a recent evening rush hour.

The ventilation system wasn't ventilating. The seats, which had been beaten into pancakes since their 1960s debut, slanted visibly toward the floor, encouraging riders' butts to surrender to the laws of gravity and slide off.

By the time the Regional Rail train reached Overbrook, several riders had asked the conductor about the smell. "The brakes burned out," he said cheerfully. "We're riding on air."

You could tell from the practiced way he said it that this was not his first time.

A Daily News reporter, whose butt felt as if it was riding directly on the rails, asked him what he meant. We were riding on the backup air brakes, he said. The primary brakes - well, you could smell their demise.

The next night, when the same thing happened, the reporter remembered, as if recalling a dream, that sun-splashed October day last year when SEPTA took journalists and rail buffs on the Silverliner V's maiden voyage - a very short excursion from Suburban Station to Cynwyd and back.

The air that day was heavy with new railcar smell emanating from the plush two-passenger blue seats - eliminating the old-school three-passenger seats with their monkey-in-the-middle torture squeeze. The ride was seductively smooth.

So a year later, this reporter, hurtling through the second consecutive night on a burnt-rubber Silverliner II, screamed within his aching head: "Was that 2010 ride on the Silverliner V an opium dream? If Silverliner Vs are real, where the hell are they? And if they're in service, why do I keep catching Sciaticaliners instead?"

There are 43 Silverliner Vs now in service - running as eight three-to-six-car trains on various lines, said Luther Diggs, SEPTA's assistant general manager of operations. An additional 11 cars are being safety-tested. SEPTA said it expects to have 66 Silverliner V's in service by March and six new ones per month after that.

But until all 120 Silverliner V's are out of Hyundai Rotem USA's South Philly plant and on the tracks, the transit agency's 73 Silverliner IIs and IIIs, which have been running since the Beatles were hot young things, will continue to harass heinies all over the Delaware Valley.

"I know, I know," Diggs said empathetically. "The ones with those flat seats? They break my butt, too."

Although Diggs has shepherded the Silverliner V project since 2006, he is not stressed by its lagging a year behind schedule, generating more than $8 million in contract-violation penalties against Hyundai Rotem USA.

"The average project like this runs three years late," Diggs said, "so one year late is not that bad. Steel was six months of that lateness.

"When we started Silverliner V five years ago, the Federal Railroad Administration required a certain type of steel to meet its crashworthiness regulations for railcars. Then the federal government bought up all of that steel from our Pennsylvania supplier to armor-plate Humvees for the Iraq war, and it took us six months to find a new supplier in Sweden.

"Then the communications supplier went under, which added another couple months to the process. We're on schedule for all of 2011, which is a good sign."

Diggs said he's able to keep his cool because long before he began his 29 years with SEPTA, he was operations manager for a nationwide trucking company, back in the day when if he lost a load of refrigerated goods because of breakdowns, his company absorbed the cost. And he'd be the fall guy.

"One time, I had a truck full of hanging meat break down on a mountaintop in Idaho," Diggs said. "Another time, I had a truck full of hanging meat break down in the middle of a desert. Had to get those trucks fixed somehow to save that meat. I did. That's stress. The Silverliner V is just running a year behind schedule. We'll have all the railcars by this summer. And then we can turn those Silverliner IIs and IIIs into artificial reefs in the ocean."