Charlie the ball bearings on the passenger car trucks are the answer, in one important respect. The reason a smooth-tired 4-8-4 can't pull a prototypical 15-car consist in O gauge, is because the physics of the prototype don't scale down well to a model. The prototype is hard to pull because of its mass, especially if there's any sort of grade. But the model is hard to pull because of the excess friction/drag. Especially when starting, if all of the cars have center rail pickups and axle wipers. Of course the loco also has less mass. But the ratio of drag to mass is much higher on the model.
If you greatly reduce the drag -- through roller bearings, etc. -- then you wouldn't need rubber tires to overcome that drag. A little wheelspin at startup is prototypical. I'll guess that if you helped the train get started- with a pusher(!), or the good old "0-5-0", a smooth-tired loco would be able to keep it rolling at speed. This is also prototypical.
Adding roller bearings to the car axles is doable, even for the home hobbyist. (It's definitely easier than replacing a grooved locomotive wheel with a smooth one!) 3rd Rail is using this type of flanged ball bearing on their diesel locomotive axles now. I'm pretty sure some high-end two rail O scale passenger cars have these types of roller bearings. That's where I got the idea. (Note that you'll NEVER see rubber tired locos in two-rail O scale.) Installing roller-bearing axles on freight cars, especially die-cast cars, would let you do neat stuff like flying drops or a prototypical hump yard.
Maybe rubber tires are a necessity for the lucky few who have 70' straightaways and O-120 curves. I'm not trying to deny you or anyone else. But I'm convinced that most of us would be better off with a no-tire option. If the manufacturers would redesign our locos in conformance with best accepted engineering practice (removable wheels, axles, and bearings), we wouldn't be having this conversation. Your loco would have a grooved axle with tires, and mine wouldn't. We would both be content. MTH already offers this choice in its HO product line. I just want the same choice in O gauge.
