Steam or diesel? Tractive rating of locomotive? Level track or gradient? Type of cars and gross weight of each?
And don't forget that, in general, model railroad curves are much sharper, and our grades are often much steeper than those of the prototype. If an overly steep grade halves your locos pulling power, putting that grade on a sharp curve will halve it again.
In real life, with big AC-traction diesels, I have (very rarely) seen 100+ cars behind one engine. For most modellers, the issue is "How long does my train have to be so I can justify more than one engine?" or "How few cars can my engine pull without looking silly?"
This is true, but the saying that "there's a prototype for everything" is also probably true. Two weeks ago, a CN train went through town with 120 cars, pulled by a single unit (something modern, looked like something from GE, but I'm not "up on" the newer locos). The line has no major grades, and many of the cars appeared to be empties. I'd guess-timate the speed to be about 30 mph. Yesterday, five modern locos, with about 15 cars. I didn't count the cars because I was driving on the highway parallel to the tracks, and I wouldn't even hazard a guess at the speed, as I was moving in the opposite direction, at about 125 kph. (Headin' for the hobbyshop!:mrgreen: )
You'll need a bigger layout. Arena-sized, most likely.
Do you ever notice that when Model Railroader does a loco test, they give the drawbar pull in ounces, then give an equivalent number of cars (free-rolling, no less!:lol: ) that the loco would pull on "straight, level track"? Who the heck has much straight, level track? Even most club layouts don't have long, uninterrupted stretches of straight track.
I checked a number of test results, and, based on the testers' observations, concluded that many good-quality locos will pull about 14 of those "free-rolling" cars, on "straight, level track", per ounce of loco drawbar pull. Based on those figures, my most powerful loco would pull 116 cars: trouble is, not much straight track, waaaay too many steep grades, and not very many of those "free-rolling" cars. :mrgreen: I run what I need to get each particular train around the layout, but, when the mood strikes me, I run what I want! By the way, Cannonball, even if you're modelling BN, your train can, and usually should, include cars from other railroads, as all common carrier, standard gauge roads in North America interchange cars with one another.
Wayne