IC 121 operated the "Green Diamond" service between 1936–1947. The "Green Diamond" began each morning with an 8:55 AM departure as Train 50 from St. Louis Union Station, arriving at IC's Central Station in Chicago at 1:50 PM. The train was then turned and serviced, for a 5:00 PM return as Train 51, arriving back in St. Louis by 9:55 PM. Seven stops were made en route, and the train ran to a 4-hour 55 minute schedule, being by far the fastest of IC's three trains on that route. Wartime in 1943 added an extra 15 minutes to the schedule to cope with traffic congestion, but the train was otherwise unchanged.