Back on TrackBy Anne MurphyIt's never too late to start over. Witness the Grand Canyon Railway, in Williams, Ariz., which restored 90-year-old tracks and turn-of-the-century locomotives to resume train service between Williams and Grand Canyon National Park. Chairman Max Biegert has invested $18 million to rebuild the historic railroad and get it rolling again. But he insists there's more than nostalgia driving the venture. With some 900,000 vehicles clogging the park each year, the railway offers alternative transportation at no cost to taxpayers, and that pleases environmentalists and park officials, as well as paying passengers. Marketing his service through regional travel agents and tour operators, Biegert predicts $6 million in 1991 sales.