On the 2nd inst., the first scream of the Iron orse was heard on the west of the Mississip. pi in St. Louis. The Pacific Railroad has been commenced, and has so far progressed that the locomotive, with burden and freight trains, is running upon it. True, only a few miles are yet in use, but who can predict how long or short the period when the locomotive will start from the Mississippi and terminate its flight on the shores of the Pacific.