Built by General Steel Castings (GSC) in the 1940s and 1950s, the Commonwealth BX (Box Express) truck could be found under a variety of express and head-end equipment (reefers, boxcars, MHC's, baggage cars, steam generator cars, etc).
Off-the-shelf freight cars were never intended to run continuously at passenger train speeds, nor were they designed to handle the frequent starts and stops of a passenger train. Under the stress of passenger train operations, standard freight trucks could oscillate and hunt - to the point of causing a derailment. Consequently, this truck was modified to include a snubber (the device which looks like a shock absorber in the center of the truck). And since these trucks are still in use today, I guess it did the trick.
Micro-Trains makes the only BX Express truck in N scale (specifically designed for their 52' REA express reefers) -
Micro-Trains - wheelbase 10.1mm, frame width 15.53mm, wheel size 33", axle length 13.7mm