David in St. Louis from St. Louis on 7/20/2009 12:42:49 PM:
I just took AmTrak to Hermann, MO on Saturday and rode to St. Charles.
As you head across the bridge, someone was nice enough to mark the bike lane (if you're on the board - thanks). It is written in yellow, and will take you there. As you come to the gas station / convenience store on your right (a BP station if I remember correctly), you will start to go up a hill (an overpass, actually). This is the Katy Trail. Like Trek said, the trailhead is on the left side of the road. There is a path on the right side, but is a little bumpy and steep.
Good luck on your ride.