https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.learntocarry.com/docs/Permitless_CCW_in_MO_SB656.pdf&ved=0ahUKEwjmnImom_jVAhUJ_4MKHUSdBygQFghGMAQ&usg=AFQjCNEmMRbXjQmxvrQFduw7FF61AOEqHw
Look at #9. Place where carry is prohibited under federal law, like post offices. The MetroLink is one of those places.
Plus, it specifically mentions an 18 year old who has not yet qualified for a CCW permit because he is not 19.