The operational limit on my layout is 4-5 cars (depending on the mix of 40 or 50 footers) because that is the size of the run around in my switching area. Now it just so happens that also matches my desire for shortline/branchline operations with small switchers and short traings. Those are the ones whose pictures I most gravitate toward in my RR coffee table books.
Now once for kicks back when i was in N scale, I did a “hook up everything and see if it’ll move” train. If I recall it was 4 SD40-2s on the front, with a GP50 and SD40-2 3/4 of the way back. About 80 cars I think. I was a beast getting one successful lap around the layout getting the helper set in the right location to not shove the cars ahead off the track faster than the lead units could pull.
Even if I had more space I’d probably stay in the 10 car range as a design criteria because of my preference for short line ops.
train length fluctuates on the short line i work for. today, we took only five cars to customers, and brought back 28 outbounds. every run is different, although it is rare to leave town with only five cars.
My prototype was running 20 4-wheeled wagons behind a 2-8-2, and needed a pusher up the 2.5% grade. By no coincidence whatsoever, I designed my railroad so the sidings would clear a train with 20 wagons and two locomotives. My passenger trains are all shorter, the longest being equivalent to 17 wagons pulled by one diesel-hydraulic loco.
On the freelanced mountain goat trail to the collieries train length is held down by the grade and hairpin curve radii, and the rather abbreviated passing sidings reflect that fact. The coal units (about equivalent to 10 JNR wagons) usually get a twelve-drivered articulated, and can’t fit in any sidings between the line’s termini.
So on the JNR side I run what my prototype ran, while on the freelance side I run what the rather small TTT locomotives can move.