The Helium Car has trucks with three (3) springs as well as outer wheel external braking. See above.
I found some MTH 3 spring trucks MTH DA-020001 with couplers on the Bay, see photo. TRIANZ has an auction for 3-spring trucks too, see:
MTH DA-0100006 Die-Cast 2-Axle 3-Spring Freight Car Truck | eBay However, the couplers are missing.
I do plan on adding a KADEE bracket for their O-gauge couplers.
Since neither trucks types have the external braking, I will make a 3D cover piece, see the green, to model the external braking.