Yes, it was on tangent track on the Portsmouth Branch. I had only one long grade on a S curve, but the radius was probably greater than six feet, and even that reduced train length. The other grades were about 20 feet long at about 2.25% and straight. My experience on two layouts over 20 years suggests that a level 18 inch radius was about equal to a 0.5% grade, but a 18 inch radius curve on a 2% grade was about equal to a 2.7% grade--the drag wasn't quite additive. At least that was the case on the earlier Pittston & DeWitt.
The late Bob Woods (aka LongTrain) and I discussed this many times, and I photographed his single modified RSD 4/5 pulling 85 cars on level track in Phoenix. There was a very slight rise on one section of the club layout, and the engine did have trouble, but managed to pull through. That run stopped with two of the four traction tires thrown off (Bob had added a pair). Bob and Victor Miranda also did some work with a modified LL 2-8-8-2, but I wasn't in on that. I believe the added weight wore out the crankpins by elongating the holes in the drivers and rods, as that engine had only one geared driver.
I found the stringline effect on 180 degree 18 inch radius curves was the delimiter going uphill (but not on 90 degree curves). Downhill without a brake at the tail, the delimiter was the pancake effect--the weight of the train would simply cause one of the front cars to lift off, turn sideways, and I'd have a sixty car pile-up at the bottom of the grade. That's why I settled on 60 cars as the max on the Portsmouth Branch--my yard's longest tracks and the passing sidings would hold about 50 cars.
All of my experiments were with three or four heavy diesels at the front end, as really good pulling steam came along a little later. A four unit diesel lashup could easily pull 100 free-rolling cars on the level without stalling.
My only problem with really long trains on NTrak layouts is the time it takes to set them up, get them out on the track, then take them down. Most folk are really impressed with 40 car trains.
