Monon wasn't the only railroad to use a "rider car". Some railroads used a regular caboose as a rider car. Monon's built the rider cars for a special purpose. I think the rider car was used on locals because of the Indiana full crew law that required and engineer, fireman, conductor, flagman, and brakeman for a train of less than 70 cars. An extra brakeman was required on trains of 70 cars or more. I assume that because the main function of a local was switching along the way a rider car was more convenient for the brakeman. If there wasn't a seat in the engine for the brakeman, he could ride in the "rider car". The rider car may have been used for LCL freight and company supplies on locals too. Princeton, Indiana (where I live) was a crew change point on the Southern Railway Louisville-St. Louis line. Eastbound trains with more than 70 cars stopped at Mt. Carmel, Illonois to allow the extra brakeman ride the 12 miles to Princeton. Westbound trains with more than 70 cars had to have the extra brakeman ride to the state line at Mt. Carmel.