Safety problems on a commuter rail line in Colorado might be holding up Sonoma and Marin counties’ SMART train. SMART officials are waiting for federal approval to start service between the Santa Rosa Airport and downtown San Rafael. The “Marin IJ” reports the hold-up might be the SMART line’s Positive Train Control safety system. Denver’s Regional Transportation District has been using the system since last year and is still having problems. Marin officials think there’s a good chance the Federal Railroad Administration is being extra-cautious until it’s sure that the safety system is free of bugs in Colorado.