The Cardinals and Royals have had a few I-70 deals over the years; here are some of the more notable transactions
1973: The Kansas City Royals traded Tom Murphy to the St. Louis Cardinals for Al Santorini. (this one is for @sighyoung)
1977: The Mad Hungarian, Al Hrabosky traded from the Cards to the Royals for Mark Littell and Buck Martinez.
1984: The Royals purchased Dane Iorg from the Cardinals (he would have the game winning hit in the 1985 WS Game 6)
1985: Royals trade John Morris to the Cardinals for Lonnie Smith
1993: STL trades Felix Jose and Craig Wilson to KC for Gregg Jefferies and Ed Gerald
2015: The St. Louis Cardinals traded Tony Cruz to the Kansas City Royals for Jose Martinez (minors).
While it was not a trade; the Cardinals also added popular submarine reliever Dan Quisenberry in 1988 ten days after he was released by the Royals.