This is a real "apples 'n' oranges" question, as even on a club layout, there aren't often long stretches of straight level track. Also, the rolling quality of freight cars varies widely. I went through some back issues of Model Railroader to gather some test specs: in general, a locomotive's drawbar pull, without the use of traction tires, is at best about 25% of the loco's weight. (I checked only those reviews for locomotives with 6 axles, all powered - this general info may not apply for 4 axle locos). Based on the drawbar pull stated in the reviews and the number of "free-rolling cars on straight, level track" that the locos could actually pull, I determined that 1 ounce of drawbar pull could move between 13.7 and 14.3 "free-rolling cars on straight, level track".
Based on a median of 14 cars per ounce of drawbar pull, my three modified Athearn U-boats will pull 348 "free-rolling cars on straight, level track", or 116 cars each.
If anybody has a HO scale mile of "straight level track" and 115 free-rolling cars, (and lives within reasonable distance), I'd be glad to drop by for a test run.
Here's a pic:
Wayne