I've done tests with grades up to 10% and currently have a long stretch of 8% grade that starts with an O40 single-turn helix. Grades of 4% to 6% are not unreasonable for most three-rail trains if you have moderate-length trains and powerful locomotives. Steep grades can have reliable operation if you avoid kinks and twists in the track. Steep grades allow more possibilities in track planning for limited space.
For the newbie people who have so many questions about how to build grades: may I suggest that you build some temporary trial layouts and you can see what works and what doesn't work with your own equipment, before you attempt to build a "permanent" layout.
Last pic shows a small trial layout to test feasibility of Lionel-type 5.6% grade. The main issue here was uphill-and-downhill speed control of typical postwar loco with spur gears. If vertical curves in the track cause derailments, just shim them to even it out.
