3-A1A
"3-A1A" means there are two trucks. The "3" truck is under the front of the unit, and has three idler axles in a row. The "A1A" truck is under the rear of the unit, and has one powered axle, one idler axle, and one more powered axle. An example is the later built FM OP800 800 hp (600 kW) railcar, six of which were built by the St. Louis Car Company exclusively for the Southern Railway in 1939.
Read more about this topic: AAR Wheel Arrangement