Last week, rail traffic came to halt Saturday afteroon after the Walk Bridge, which carries Metro-North Railroad and Amtrak trains, failed to open and close. The bridge had been scheduled to open for a sailboat to pass about 2 p.m. ConnDOT officials said it appears that the bridge failed to open due to the thermal expansion of the centerpiece moveable span. The latest malfunction comes as ConnDOT moves forward with an estimated $700 million plan to replace the bridge, build a rail dockyard and make track improvements in Norwalk.