Amtrak is perfectly safe. There is nothing to be concerned about.
St. Louis Metro has had some personal safety issues on the light rail, but they have responded aggressively and things have improved a lot. I ride it regularly with no fear.
All of this is off-topic for a UA board. Someone here can probably look up the exact stats, but I think the chances of a flight being canceled out of St. Louis in June are probably less than 1%.