Throughout the war the St. Louis Republican newspaper ran advertisements of the schedule of the North Missouri Railroad, along with a map providing the route as well as the stopovers along the route. Right now I am looking at one such advertisement providing the "Winter Arrangement" for 1861-1862. The map shows the route from St. Louis to St. Joseph ("The only all Rail Route to the Far West"), along with the 30 stops along the route and the distances between each one. As of 9 Dec. 1861 the St. Joseph Express would depart St. Louis at 1:30 a.m. at the depot located at North Market Street/Second Street, and would arrive at Macon at 10:30 a.m. At Macon the NMRR connected into the Hannibal & St. Joseph Line, and would arrive at St. Joseph at 9:09 p.m. Another scheduled passenger train would leave St. Charles at 4 p.m. and arrive at St. Joseph at 9:15 a.m., while a third scheduled train, this one carrying passengers and freight, would leave St. Louis at 5 a.m. and arrive at Macon at 8 p.m.

I don't have information at hand regarding how the scheduling might have changed during the course of the war and the resulting troop build-ups and material shipments--but that was the train schedule for the first winter of the war.

Regarding the route between St. Louis and St. Joseph....The scheduled stops between St. Louis and Macon, in order of arrival, included St. Louis, St. Charles, Dardennes, Perruque, Millville (Foristell), Warrenton, Jonesburg, Florence, Wellsburg, Martinsburg, Mexico, Centralia, Sturgeon, Renick, Allen (Moberly), Jackson, Macon.

Stops between Macon and St. Joseph included Macon, Callao, Bucklin, St. Catharine, Brookfield, Laclede, Chillicothe, Utica, Breckenridge, Hamilton, Cameron, Osborn, Stewartsville, Easton, St. Joseph.

The first Federal to reach Centralia after the massacre, Col. D. P. Dyer, was in St. Charles when he word reached him. He immediately commandeered a train and balled the jack to Centralia, stopping in Mexico just long enough to pick up two companies in his command, the 49th Missouri Infantry. Once in Centralia he loaded up the remains of the men who had been taken off of the train and executed, taking them back to Mexico for burial.

Anyway, all of that is the long way to getting to the short answer to your question, which is yes, the soldier belonging to the 17th Illinois Cavalry could very well have boarded the train at Mexico.