@Mellow Hudson Mike I don't have any track set up right now, so I can't post a video. But if you come to my house in Fort Worth, TX, I'll gladly set up a loop to show you.
The vast majority of people on this board have NEVER run a properly geared 3-rail loco, because so few exist. Especially stock out-of-the box. But the 1,000,000 modelers who were in 2-rail O and every other scale (and I mean before the advent of DCC decoders with Back-EMF, etc.) would disagree with you. I strongly encourage you to experiment and enlighten yourself...
One of the easiest locos to obtain is an MTH RailKing 0-4-0 docksider from circa 2001-2002. This is a chunky all-metal toy train with a tiny, cut-rate motor. But it has a big flywheel and a favorable gear ratio. Try it on O31, or even O27. You'll see- it WON'T slow down going from straight to curved track. It will also creep consistently at about 6 scale mph or less (assuming you have fairly uniform track voltage, no pinches in the gauge, and no wide joints that would snag wheels and rollers.) And it will do that running light. Maybe even better pulling a string of cars!
Another loco that's currently produced is the Bachmann-Williams 4-6-0 Ten Wheeler. It can be had for under $200 street price. I'll bet John has never seen one of these on his bench. Why? Because there's very little inside of it that might fail. AND they run so well stock that there's no need to "upgrade," add speed control, etc. They just don't need it for smooth operation! 40:1 or bust!
Some 3-rail trains use chains or pulleys to transfer torque from the motor to the wheels. Even before Lionel introduced the Odyssey motor, I found that by changing the ratios I could achieve very slow and consistent operation on par with the best HO diesels. With enough RPM and flywheel mass, the loco will utterly ignore curves and *small* grades (such as a mismatch in thickness across the seam between adjacent plywood surfaces.) Now, if you want to climb a Lionel #110 trestle set, it will do that too, and even maintain a controlled speed on the way down! But like a real train it will slow a little going uphill and speed up a little going downhill. If you need absolutely constant speed in that scenario, you could create an insulated block, and install variable resistor(s) in series with the track feed.
Maintaining a set speed on graded track is probably the best "use case" justification for closed loop speed control. It's NOT a blanket exemption from sound engineering practices like BIG flywheels (maybe more than one) and gearing that puts the motor in the linear part of its RPM-voltage curve at a scale coupling speed of 4 MPH. IMO the RIGHT way to build a train is to gear it low, and attenuate any resultant NVH. This is quality, and yes, it adds cost! A Hudson geared for 100 scale MPH? How big is your layout? How many seconds will it take before it travels out of your sight? Arguably slow speed is more important on small layouts than on large ones. And the sharper the curves, the greater the need for low gearing because of the friction problem discussed above. What the world needs (and doesn't know it) is a toy sized train that runs like a fine scale model.
Speed control gets bundled with almost every loco today because most hobbyists nursed in 3-rail O are willing to pay a premium for "features." I could make a case that Lionel O peaked in 1939. Smoke was a gimmick. Magne-traction was a gimmick. And speed control is a gimmick too. I'm grateful for this opportunity to explain to the community at large that there IS an alternative, a road not taken. And the best defense against buyer's remorse is an enlightened consumer.
