A steel-lined boxcar of low-level radioactive waste returned to the Rocky Flats nuclear weapons plant near Denver after being rejected by Idaho, leaving in its tracks troubling questions about how the nation handles its atomic leftovers. The boxcar, ordered out of Idaho by Gov. Cecil D. Andrus on Saturday, was shunted to a siding at the Rocky Flats plant 16 miles northwest of Denver while officials discuss their options. Colorado Gov. Roy Romer turned down a Department of Energy request to expand storage facilities for low-level radioactive waste at Rocky Flats and asked that the Department of Energy not unload the rail car. The state now allows storage of up to 1,600 cubic yards of radioactive waste at Rocky Flats, and the plant legally could accommodate the boxcar's cargo, officials said.