I agree with blcjr, an image or your specific cover would be most helpful for a detailed answer. A typical airport dedication cover would have been canceled at the airport then sent onward by truck, rail, air, or hand-back as appropriate depending on the destination.
More broadly, the R
ervice oversaw more than just mail handling by rail. They managed and coordinated the ground/air transport at depots, airports, truck terminals, and highway post offices too.
Circa 1950, the RMS transitioned its name to a more general P
ervice - thus PTS in the killer in later years.
Here are two first flight covers from the airmail field in Indianapolis with RMS and PTS in the killers in 1949 and 1954 respectively.
Bottom line: don't get too caught-up in the "RMS" part of the marking.