I assume you're talking about bike races -- surprisingly, I am not aware of any. Other than the Race Across America, which uses a small part of the Katy Trail, but you have to ride the other 3000 miles to participate in this coast-to-coast race!
On the other hand, there are usually a few running races on the trail each year. The
Lewis & Clark Marathon uses part of the trail near St Charles each September, and there are usually a handful of other 5K/10K type foot races every year.
If/when I learn about any races, whether running or cycling, I'll add them to the
Events Page.