Railroad and other officials were straining this week to explain why four boxcars of uranium were trucked through the middle of St. Petersburg from the suburban stations of Avtovo to Kapitalovo, where they were eventually impounded. The boxcars, carrying an unspecified amount of uranium ore, were initially shown Tuesday by TNT television as they wended their way through the center of St. Petersburg from Avtovo in the north to Kapitalovo in the south, a journey of some 20 kilometers.