Hello. We’re trying to get our new layout going over the holidays. It’s an around the walls n-scale layout with a duck-under using kato unitrack. What are some recommended “best practices”… 1) how long of a straight should one have on either side of a plate girder or truss bridge before a curve? 2) how long of a straight should one have before entering a curve from a turnout (the main route, not the diverging route) 3) how long of a straight should one have in an s-curve (the length of the straight between the change in direction of the curve? Thanks Mike
It depends on what you have running into and out of the bridge, and what construction the bridge is. A through truss bridge may be too narrow for longer steamers with a lot of rear cab overhang if the curve starts too soon after the exit of the bridge. A little SW/NW switcher diesel won’t be bothered, but a C&O 2-6-6-6 could have severe clearance problems. Ideally, the engine would be almost aligned with the track axis across the bridge when the front axle crosses over the bridge shoes. If you can’t do that, try for half the length of your longest engine before the curve starts, somewhat less on a well eased curve.
I had a turnout very soon after a Pratt Truss bridge on my first layout. The points were maybe 2" from the leading truss members. One of my steamers scraped part of the bridge, so I had to thin the plastic a bit on the one truss member.
Through experimentation, you are likely to find this solution, and it applies to the previous question as well. A well eased curve could perhaps start as soon as the turnout ends…at the first join…but if you move directly to a fixed radius, you would want a minimum of something like 1/2 the length of your longest engine, maybe less. Mock it up and check to be certain.
The general rule is the length of your longest item of rolling stock…but check for yourself. Body-mounted couplers on long passenger cars will almost certainly need one full car length, whereas shorter cars will need less,
Thanks for the reply. What makes our layout difficult is a combination of a) I’m still a newbie and my partner is only 10 and b) We’re pretty eclectic on what we run… from switchers to Big Boys, from F3’s to Shinkansens to double stacks. So I think the general rule of thumb is “if the train is on a curve, you need a straight before you change directions” 
Sounds like a pretty aggresive layout for a “newbie”. If you are using the Kato unitrack, you should be fine with the sectional pieces. I would recommend a full section of straight around turnouts esp, as they are trouble spots for derailment. And you may want to lay it out with the track and test your longest cars.