I can have as many as 7 unit trains on my layout at any given time, 2 on the 2 main lines and 5 on the sidings in the various sections of the layout. This photo shows 5 unit trains in the middle section of my layout:
And, I just thought of another unit train I can make that I never knew I Had until this moment: livestock, which I will run and videotape later.
I can imagine seeing massive real train yards in the Chicago or NYC metro areas where there could be over 100 unit trains. Arnold