Among other applications, these heavy duty six-wheel trucks were used by the Santa Fe on their "big dome" and hi-level diner cars. Kato and Bachmann both make N scale versions of this design.
The Kato version was made for their UP City of Los Angeles storage-mail and Santa Fe El Capitan dining cars -
Kato catalog #800122G (6-Wheel Truck, Gray, w/ Gray coupler - Clip mount, 1 ea).
Kato catalog #800122 (El Capitan 6-Wheel Diner Truck, Silver, w/ Silver coupler - Clip mount, 2 ea)
Black versions of the Kato trucks were also used (incorrectly) on their Milwaukee Road Olympian Hiawatha Super Dome cars -
The Bachmann version was made for their 85' Full Dome Coach -
The Milwaukee Road used a similar (but not the same) 11' three-axle GSC truck on their Super Dome cars. I'm not sure what it's official name/designation might be or if it was used on any other cars besides the domes -
Micro-Trains makes this style of truck for Intermountain's Super Dome cars -