What is the maximum slope for O Gauge?
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AFAIK, there is no set maximum as it all depends on the engine, how many cars it‘s pulling and how well the grade is constructed with easements, etc. The graduated trestle sets sold by Lionel and MTH are designed for around 5%, but most modelers like to keep it to 2% or less while some are okay with 3% or so. The steeper the grade, the fewer cars you can pull and the less realistic it will look.
https://ogrforum.ogaugerr.com/...c/recommended-grades
And everybody welcome our new member! He's a fraternity brother of mine...
Jon 
Welcome @tkcarson!
The rules aren't hard and fast. Smaller slopes look better and bigger slopes require an engine with better pulling ability. Just don't forget to add a transition to your slope to help thing run smoothly.
On my layout I used a Lionel trestle set from the 60's as a guide, then doubled the spacing between the bents, and created filler supports from wood. Slap on some retaining walls from Woodland Scenics, and nobody would ever know what ugliness lurks beneath!
Jon
If you're going for a prototypical look then less of an incline is better. However unless you own an empty warehouse or a MacMansion most of us don't have that much real estate available. Also remember that curves at the bottom and/or top of a grade will reduce the available incline. I had to settle for a 4.17% grade over about 20 feet as a max. This allows me to run a single motored RK Alco PA pulling a dummy RK B unit, an express reefer, and 6 KLine 13" passenger cars up it. However a two motored MTH Alco RS3 will not pull 12 NMRA weighted Weaver hoppers up this without a head start and some wheel spinning. It all depends on what you want to achieve and want your finished railroad to look like.
I have a 2.5% grade over around 24 feet, part of that is the easement at each end to accommodate large locomotives. That being said, i don't have any excessive running restrictions, here's a case in point.
This is a 54 car tank train pulled by a single USRA Mallet locomotive, it has no problem with the grade.
