The Postcard shows a view of the First Congregational Church in Detroit, Michigan where a Mr. Atkins was the former Pastor. (Evidently he had lived in Rhode Island at one time and was known by Mrs. Claflin.) The postcard is canceled with a 1910 Detroit and St. Louis Railroad RPO, (Train Nr. 9). The stamp is the 1-cent (Green) Franklin issue of  1908-09, (Scott #331).

View: The Face of Postcard - First Congregational Church 

RPOs or Railway Post Offices were operated on special mail cars. The mail was actually sorted and canceled on board the train between destinations.  

I could find no record of a Detroit and St. Louis Railroad and it must have been a branch line of another major railroad, possibly the "Norfolk & Western." My written records are of New England Railroads only.