On my critters - it hasn't been impossible to power a 4-wheel truck, but it's been damn difficult to make one work electrically.
First of all, you have a four-legged stool that unless it's equalized - only three are actually touching. Any track warp of any kind and one wheel is slightly off the rails. So the tiniest spec will stall them, let alone a switch frog.
As I built my first ones - the EMC 40, getting it to mechanically equalize was a bigger deal than getting it powered.
The Kato truck is better than most as it is designed to equalize - but only if there is spring pressure down on the ears. Without that, it's just another four-legged stool. The Kato 11-103 and 4 don't have pickup ears - uses wheel wipers, and doesn't equalize at all. That's whats under my Trackmobile. It pulls like crazy and is nice and slow, but pickup? Forget it. That's why I added the 'scale test car' on the back as an electrical pickup tender - a leftover 11-105 pickup truck. That did it. Worked great.
/>
The GE 25-tonner was designed to equalize off the Kato 11-105 truck that Ron used and to this day, is a nightmare to tune. It actually has fine brass wires inside the truck to spring the wheels down and keep them in contact, and the pickup ears are free-floating. With Electrofrog turnouts, it actually runs pretty well.
I've done two more 25-tonners since with RP printed shells and the TU7T mechanism.
/>
I'll even do custom builds on that, its turned out to be repeatable - but it's no electrical miracle. It's not as good as the Kato version but it's still pretty good. The body shell from Shapeways works just fine.
The absolute tiniest thing I've ever made that runs is the Burro crane, off the Modemo one-axle drive chassis, bored into the inside of the REA burro crane kit. Same thing, I could get it to run but electrically it was a no-go. I permanently married it to a 40' flatcar (loaded with rail) with Kato caboose trucks under it for pickup, and its quite reliable although with one axle power the 40' flatcar with a rail load is as much as it can move.
/>
So if you go this way, you need powered frogs, and a creative way for pickup, but it can be done. The mechanical part is easier than the electrical part.
