Last year was an unexpectedly auspicious year in the history of public transportation in Los Angeles. Transit ridership -- bus and rail -- rose to 497 million boardings, a level not seen since 1985. That means less traffic congestion, stronger revenue for the Metropolitan Transportation Authority and more poor people without cars getting around the city.

But there's a wrinkle to this success. Since the mid-1990s, the transit agency, on court orders, has reinvested in its bus service, and this has helped bring back riders. Unfortunately, it may get no better from here on because of the agency's goal of building a rail network.

This is not the first time the MTA has faced this problem. In the early 1980s, bus boardings soared because sales tax revenue generated by passage of Proposition A in 1980 was used to subsidize fares at 50 cents. But in 1986, this money was shifted to rail projects, initially for the light-rail Blue Line that runs from downtown Los Angeles to Long Beach. It was the first step in a long-term effort to reintroduce rail transit into L.A.

With the subsidy gone, bus fares rose sharply and ridership dropped. Blue Line service began in July 1990,but total bus and train ridership continued to plummet, reaching a low of 364 million boardings in 1996.

In late 1996, the MTA settled a discrimination lawsuit brought by minority and low-income bus riders seeking improvements in bus service. Rather than continue the trial, the agency accepted the terms of a federal consent decree negotiated with the Bus Riders Union. Over the next 11 years, it added buses, started new lines and held fares in check to improve the country's most overcrowded bus system. As a result, users of public transit gradually started to increase again. Yes, some chose the Blue, Red, Green and new Gold rail lines, but the majority of riders returned to buses. Most of the new rail riders were former bus riders forced to use trains because of bus service changes. And others were attracted by low rail fares and the added bus service that got them to and from the rail lines.

Paradoxically, the MTA's rail projects, which required fare increases and reduced bus services, have cost the transit system riders. Using MTA data, our analysis indicates that they produced a drop in train and bus ridership of more than 3 billion boardings from 1986 to 2007.

Although we've now gotten back to 1985 levels in terms of public-transit use, the county population has grown by more than 2 million since then. That means, on a trip-per-capita basis, the transit system is still not performing -- by 20% -- as well as it did 22 years ago.

The MTA says it has spent $7.1 billion on its Blue, Red, Green and Gold lines through June 2006.But this number ignores billions more dollars in associated expenditures, such as $100 million for Green Line rail vehicles, $153 million for the canceled Red Line extension to East Los Angeles and the canceled extension from Mid-City, and $899 million now being spent to extend the Gold Line to the Eastside. Phase I of the Exposition Line from downtown Los Angeles to Culver City will add more than $800 million to MTA spending, and Phase II of the line will cost at least $1 billion. The $7.1 billion also excludes $1.5 billion to operate the rail lines for the last 18 years and more than $4 billion to maintain them through 2025. And don't forget the billions of dollars in interest on borrowing to build the lines.

Taking all this into consideration and adjusting for inflation, the MTA has spent more than $11 billion since 1986 to build its rail network, and the effect has been to reduce total transit ridership on the system by more than 3 billion boardings. That's a bizarre result.

And transit service is certain to get worse. MTA data show that the median household income of its riders is less than $15,000 annually. Their transit choices are very sensitive to fares. But to help pay for its continued rail expansion, the MTA will have to raise fares as high as politically possible, then cut service and routes if ridership drops in response to the increases. Freed from federal court oversight in October 2006, the agency increased bus fares in July. Since then, ridership has dropped by 5%. More fare increases are scheduled for July 2009.

The politicians who sit on the MTA board should be held accountable for this cynical strategy of pursuing rail lines at the expense of overall public-transit use and on the backs of low-income, bus-dependent riders. Bus fares and transit investments should promote not discourage transit use.

James Moore is chairman of the Daniel J. Epstein department of industrial and systems engineering and director of the transportation engineering program at USC. Tom Rubin is a transit consultant.