In mid-April, The first freight train emerged from an underground trench a few miles south of downtown Los Angeles and headed along a separate right-of-way toward the enormous rail yards east of downtown Los Angeles. This is the kind of event that would seem to be a major breakthrough of the 19th Century, not the 21st. But the switch giving the freight train the green light was pulled by U.S. Transportation Secretary Norm Mineta, California Gov. Gray Davis, and Los Angeles Mayor James Hahn in front of a crowd of more than 1,000 people. And, in general, the political leaders heralded the opening of the Alameda Corridor with the kind of rhetoric ordinarily reserved for space launchings — or at least freeway openings. Davis compared it to the opening of the transcontinental railroad. Rep. David Dreier called it "the silk road of the 21st Century." This rhetoric might be a bit extreme, but you cannot deny that the Alameda Corridor is worth talking about as a piece of economic infrastructure. It is a good example of the most important kind of economic development project our metropolitan areas will see in the 21st century: the missing piece. In scale, cost, and complexity, the Alameda Corridor rivals the most ambitious infrastructure projects of its time, including the Central Artery highway relocation effort in Boston. It's a $2.2 billion project that required the combined efforts of dozens of federal, state, and local agencies, as well as two major railroads, Burlington Northern Santa Fe and Union Pacific. It took more than 20 years to bring the project to fruition, five years for construction alone. Underneath all the complexity, however, the Alameda Corridor is a very simple idea. It is an enormous trench -- 50 feet wide, 33 feet deep, and 10 miles long — combined with a series of bridges, overpasses, and underpasses at either end to create a separate, 20-mile-long right-of-way for freight trains. Its purpose is also simple. The corridor is designed to slice through one of the most crowded and congested parts of Los Angeles and remove one of the biggest roadblocks to the efficient movement of goods in the United States: the gap between the containerized ports in the Long Beach area and the transcontinental railroad system that begins in downtown Los Angeles. The Alameda Corridor will double train travel speeds through the corridor and cut shipment time from three or four hours down to 30 minutes. There is little question that the national economy will benefit from the corridor project — as will the ports of Long Beach and Los Angeles, which together make up the largest port in the West. For years, however, a debate raged over whether the other communities along the corridor would be winners or losers. The corridor passes through some of the most densely populated communities in the Western United States -- mostly Latino and African-American working-class suburbs like Huntington Park, South Gate, Lynwood, and Compton. These communities have some of the lowest incomes, educational levels, and home ownership rates in the West. The presence of the rail lines helped to create southern Los Angeles County as an industrial powerhouse, but it has also put these communities in the path of pollution, noise, and danger. Throughout the corridor's planning, these communities feared that they would bear the brunt of more train traffic and yet miss out on the resulting jobs. There are still outposts of community opposition to the corridor, but they are rare. Speaking in April on the radio program "Which Way L.A.," Carlos Porras, executive director of Communities for a Better Environment, complained that "these communities have been scapegoated again in a tradeoff for regional benefits that compromise localized impacts." But he was hard-pressed to provide specifics. And even the most critical elected officials along the corridor signed onto it in the end. By and large, political opposition was blunted by two things: a program of local hiring that politicians have described as aggressive and the creation of the 10-mile trench. At the April grand opening, the elected officials repeatedly boasted of the following job numbers: About 1,300 local residents were employed in corridor construction. Six hundred of these workers entered union apprenticeship programs. And about 400 young people worked on the project as part of the California Conservation Corps. For a $2.2 billion project, these numbers are tiny. And many of the jobs were temporary; indeed, most of the permanent jobs are located not along the corridor but at the ports themselves, where the amount of cargo is expected to triple during the next 20 years. Nevertheless, the local hiring effort gave politicians along the corridor something to praise. For the communities, the trench is a bigger deal. Boxing in the trains and pushing them downward transformed the corridor into the railroad equivalent of a flood-control project. The communities are protected from many of the adverse effects of trains rolling through their neighborhoods. For most of its length, the trench is not pretty. In keeping with the flood-control theme, it looks a lot like the channelized Los Angeles River — a large, depressed box of concrete along Alameda Street, fenced off so no one can enter it. That does not seem to bother local politicians, for whom a trench is far preferable to an endless series of freight trains snaking over the surface of their towns. Like most American cities today, Los Angeles is no longer a freewheeling place with plenty of room to grow. Like the trains along the Alameda Corridor, it is boxed in. In Los Angeles — as in Boston, New York, Chicago, and elsewhere — the future of economic development lies not in building brand-new things. Rather, it lies in filling the gaps and making existing systems more efficient. As the Alameda Corridor proves, the missing piece does not have to be complicated or beautiful. It just has to work.