The chatter of hammer against spike and the scuff of rails against wooden crossties mark time in the west end of Simi Valley, where Southern Pacific railroad workers do maintenance work on the tracks. The workers, part of a crew of about 50, will clank and yank all the way to Ventura. They pull up sections of rail, covering about a mile a day. They repair the underlying crossties and rail anchors and then replace the rails, cleaning off the debris. When a freight train approaches, the workers must replace the rail and pause until the train lumbers past. Then the workers pick up their tools and resume their noisy work, in a routine established in the West more than 100 years ago.