I travel to St. Louis several times a year with Rosalyn, so I have a few suggestions.
The station Grille Restaurant is located in the St. Louis Union Staton Curio hotel. This is a great breakfast restaurant. It has one of the best buffets in the country for breakfast. We rarely meet up officially as a group for breakfast, but this restaurant is worth a visit unofficially.
https://www.yelp.com...source=(direct)
This next restaurant, Lombardo's Trantoria is across the street from the Curio Union station hotel, about a 15 minute walk from the metrolink Union station stop. I have not eaten here but it might be an alternative that is closer to the Amtrak station. If 21 happens to be running late, we could wait in the lobby of the Curio hotel. There is a cool light show on the ceiling of the lobby that plays every 30 minutes.
https://www.yelp.com...source=(direct)
These next two are next to the Laclede's Landing metrolink stop. The first is old Spaghetti factory, a railfest classic. https://www.yelp.com...source=(direct)
The other restaurant is nearby and a possiblity. It is the Morgan Street Brewery. https://www.yelp.com...source=(direct)