@Bob Delbridge If the 'J' is really geared at 40:1, a scale speed of 54 mph is the most you can expect, unless you want to drive the motor above its nominal 12-volt rating. Here's the math:
Mabuchi RS-555 motor rated for 12,000 RPM at 12 volts. 85% of that is 10,200 RPM with a typical train load. 10,200 motor RPM / 40:1 = 255 driving wheel RPM. 70" drivers times 3.14 (Pi), times 255 = about 56,000 scale inches per minute, or 3.3 million scale inches per hour. Divide that number by 12 inches/ft = 280,000 scale feet per hour, and divide by 5280 ft/mile = 53 scale MPH. IMO plenty fast on a smaller layout. Overdriving the motor at 15 volts probably won't hurt anything, and would get you close to a passenger track speed of 60 mph.
You might find that your Mikado is geared around 25:1 with smaller driving wheels in the neighborhood of 63 scale inches. So ironically it would be a little faster. So what motor is in the Lionel 4-8-2, and what's the gear ratio??
I recall that in the early 1990s, Andy Edelman (now with MTH) had an "upgrade" service for Northerns, where he replaced the stock Mabuchi motor with a Pittman, and also swapped the gearbox to the one used in the Mikado, I think. (Both of these locos were imported from Samhongsa and had a similar design.) The net result was a faster-running Northern that drew less current, because the Mabuchi motors were current hogs, especially at higher RPM. I couldn't afford Williams brass at then-current prices, but if I did I'm pretty sure I would have left mine stock! Unfortunately the "faster is better" mentality took root at MTH, consequently none of their first-generation locos ran as well as the original Williams and Weaver brass, until the band-aid of speed control was applied circa 2000. The rest, as they say, is history!
