Frisco-series 2100 equipment consisted of self-propelled rail motor-cars, mostly gas-electric models, with a few gas-mechanical models given 3000-series numbers. These railway vehicles were commonly known as “Doodlebugs” for their insect-like appearance and the slow speeds at which they would doddle or "doodle" down the tracks. These were used to service various low-volume branch lines in the Frisco organization. An initial order for ten was placed in 1910, with seven more arriving by 1913, putting Frisco in the forefront of gas-electric operation at that time. The initial batch, numbered 2100 to 2109, included nine baggage-coach combinations, as well as one baggage-mail-coach unit. Frisco's peak year for motor-car mileage was 1931, and its fleet at that time included twenty-three gas-electrics, five gas-mechanical cars, four trailer coaches, and six mail-baggage units. The final Frisco run of a Doodlebug was on November 8, 1953, when No. 2128 traveled from Ardmore, Oklahoma, for the four hour trip to Hugo, Oklahoma.
