Train length and layout design
How you design your layout's passing sidings, yard tracks, and return loops can have a major impact on train length. In this tip we discuss briefly what the issues are.
You can have trains longer than passing sidings and yard tracks and can deal with them, although they will slow things down a lot. But a train longer than a reverse loop will not work -- the laws of physics says two objects cannot occupy the same space at the same time — so the front of your too-long-for-the-reverse-loop train will collide with the back of the train once it reaches the end of the loop!
With passing sidings, the key is opposing trains. As long as one of the two opposing trains will fit, the other train can be any length — you can get the trains past each other with a bit of advance planning. But if both trains are too long for the siding, you'll need to do a double-sawby maneuver. Such a maneuver is fun once in a while but it would get old if it was standard operating procedure. This means passing siding length has a major impact on train length if you want opposing traffic to flow smoothly.
With visible yards on the layout, a too-long train can always "double the yard", which means you split the train in two and it takes two yard tracks. This practice is not uncommon on the prototype so it could be an acceptable operating procedure as long as you are aware that it will slow down yard operation somewhat.
Finally, too-short staging tracks can be a problem, since it will make you "double the yard" in staging as well. Requiring trains to do this in a visible yard on the layout is one thing, but requiring this in staging is going to get old, so the recommendation is don't. Staging track length has a major influence on train length.
Layout design analysis article on Joe Fugate's Siskiyou Line web site