Earlier this month, the six-year-old Alameda Corridor marked a six-digit milestone. The Los Angeles-area intermodal corridor logged the 100,000th train to use the high-speed freight-rail expressway since it opened in April 2002.   The 20-mile corridor connects the ports of L.A. and Long Beach with downtown L.A. rail yards and the national rail system. The Alameda Corridor Transportation Authority owns and governs the corridor, which includes a 10-mile, below-ground and triple-tracked mid-corridor trench shared by BNSF Railway Co. and Union Pacific Railroad via trackage rights.