North American freight railroads collectively use a computer-controlled servohydraulic test bed called the Simuloader to perform full-scale multiaxial fatigue tests of railcars at accelerated rates. It is used to verify analyses, reveal design deficiencies, test modifications and provide safety from failures not accounted for in standard specification tests. While material coupon testing can address very precisely and directly the science of mechanical fatigue, full-scale testing is often used to address gross design, manufacturing and production-related fatigue issues. The input waveforms used to animate this machine are typically created from combinations of both time histories and cycle counted data. Simulations are accelerated through the utilization of only those events that produce significant stresses on the structure. This paper describes the test bed and its vehicle interfaces, the stress analysis of a test car, acquisition of service input and car body response data, drive file development techniques and the iterative process of tuning the machine/railcar system to improve global response accuracy. The complex data gathering and reduction process associated with creating and tuning input drive files for this railcar test bed is illustrated using data from experiments recently performed by the railroad industry.