SACRAMENTO  The western terminus of the first transcontinental railroad is a forlorn place these days. All that is left of the historic railyards are seven immense brick buildings, the remnants of an era when this site was the busiest industrial center in northern California. There is little indication yet of a $5.3 billion redevelopment plan to transform the site.

But the old railroad buildings are expected to come back to life as the retail portion of a huge project called the Sacramento Railyards. In the next 20 years, the developer Thomas Enterprises, based in Newnan, Ga., plans to build up to 12,000 residential units, 800,000 square feet of commercial space, scattered small parks, a railroad history museum and a performing arts center.

The ambitious housing plans reflect the anticipated growth of Sacramento, a city of 481,097 people that is expecting 100,000 new residents in the next 20 years. Thomas Enterprises says it intends to build many of the first 400 residential units in mixed-use projects, with retailing at street level and housing on upper stories. In later phases, the plan calls for more units, this time in a mix of low-rise and high-rise apartment buildings.

Much of the commercial appeal of the project results from its proximity to downtown Sacramento, particularly the central business district and the State Capitol complex, which lie south of the old train yards. When completed, the old Union Pacific property will become an extension of the downtown, effectively doubling its size.