The OP only asked about freight trains so my first reply was to that. My passenger trains include everything 2 car commuters to the 9 car Walthers 20th Century Limited set. My favorites are the 3-4 car local passenger trains with a mixed consist of heavyweight and lightweight cars. I think those have a lot of character. My new branchline will have a doodlebug.
My shortest train is a passenger train made up of 6 Roundhouse Palace Cars being pulled by a 4-4-0, while my longest train is a freight made up of 40 freight cars (good micture of 50’ and 40’ cars) being pulled by a F7 ABBBA set.
I currently am running a switching shelf layout. I have run-around tracks at each end of the layout that can handle about 10 40’ freight cars, so the normal train is about 8 cars with caboose. I have one town in place, and am adding another (track and buildings in place, but no scenery yet); each town has industries that can handle 8-9 freight cars at max.
Eventually there will be reverse loops added as the layout continues to expand, so eventually I will be running passenger trains up to 7 cars, and iron ore trains around 30 cars long.
On my 4x8 table-top pike I have run a 4-unit consist of GP40’s with twenty-four 32-foot hoppers and a caboose.
I admit that it looked absolutely unprototypical. There was less than two cars distance from the lead loco to the caboose.
Typically I run between 4- to 16-cars per train. I have a 4% grade to get up to the coal unloading platform. On that grade I run a max of 8 hoppers with a consist of 2 GP30’s and a helper GP30.
On the mainline; consisting of 18- and 15-inch radius snaptrack, I run the longer 16-car coal drag.
Because of the tight radius curves I only run GP’s or 4-axel switchers.
For the Olde Tyme excursion train I run a USRA 0-6-0 with a Vanderbuilt tender and two 47-foot passenger cars and a bobber caboose. This is able to negotiate down the 15-inch radius corkscrew trestle.
Don’t have a layout but plans are for “prototypical” lengths and engine sets. 1964-1970 SP&S & GN/NP power consists, I need to get some GN GP9’s, WP, UP and SP units as well. More foreign cars as well.
At the Pasadena Model Railroad Club, we frequently run 30-50 or more freight cars, and passenger trains can go 10-20. Here’s a hundred car train being pulled by a scratchbuilt UP coal turbine. It was fine in the flats but it all went a bit pear-shaped when we hit the 2% grade… A couple of broken couplers and one stringline. A couple of mid-train helpers got us up the hill.
My little HO scale switching layout is only about 20 ft long, point-to-point with about 4 feet of that as staging (will use cassette or sector plate to maximize this in the future). My trains are no more than about 8 or 9 cars maximum, one engine and a caboose.