STOCKTON — A plan to expand the Altamont Corridor Express farther south into new territory while speeding up service between Stockton and the Silicon Valley would have an economic impact of $1.14 billion through 2029, according to a report commissioned by rail officials seeking to expand and improve service on the rail line.About half of the impact, or $565 million, would come from the net benefit to the region brought by buying equipment and paying to build the project, [...]