Prototypically, it's really steep. The White Pass and Yukon has some 4% grades with some sharp curves .... the speed limit is low, and it's really kind of extreme trackage. There are grades on the Durango and Silverton that range in the 2's ... and that happened on a lot of other narrow gauge railroads as well. Logging stuff has grades even heavier than that .... there's one grade coming into Whittaker station that's around 10%.... and it looks like a roller coaster from the train. Geared steam is the rule there ... apparently they tried it with their 45 tonner once and it had trouble making it up, light. Mainline and other Standard gauge stuff ... unless you're talking about a siding that couldn't be helped.... 1% is generally considered a big grade (of course there are exceptions and mountainous routes with that and more.)
In the model world ... a 3% grade on a curve will limit your train length because both the grade and the curve will add load to the train, one by gravity and one by drag. But, generally if you keep them under 4% you'll find it's at least manageable. If you want to run long trains ... and avoid wear.... don't.
but dont listen to me, my layout is decidely different approach than anyone elses here. I run very short trains with very short cars, pulled by very short engines. If your doing more big car, big engine, mainline type modeling I would try to keep the grades down too. 3% may be good for a straight, but on a curve I would try to limit it to 2%.
Great place for helper service on longer trains. Just have a siding for a helper to sit on between trains and ease it out behind to push up the hill. Cut off on top and drift the helper back downhill to await the next train. Operations!!!
I ran recently at a friend's place where he has a 3% grade on a curve. My Shay was doing fine until we came up after a live steamer, without having scale sand, I ran out of traction due to water, oil, etc on the track....
So we initiated helper service...made for great operations
I found this problem when my Nephews were here and I ran trains. I made a long train pulled by two SD45's. It stalled at the top of this grade. Turns out the grade was almost 4 %. There is a bridge involved and a curve. There is a HUMP at the west end of the bridge which adds to the problem.
The bridge was not one of my better achievements. So I build a new bridge and installed it with a 2.25 % grade. That means I had to change the approach on the west end of the bridge. I been working at it and working at it. Now I question the accuracy of my Calculating level.
The frist bridge I did this I used a 4 ft piece of squar tube and a step wedge graduated in 1/4 inch steps. Well something went wrong there.
Well not I got the new bridge at a 2.25 % grade and the approach track at 2.2 or some where in there. I re did my approace after I asked the question here.
I will know more sunday after I finish the curve. I will keep you posted.
My indoor layout has lots of 4% grades, some on curves. This limits train length so a few cars at most depending on the loco involved. I also have an R1 spiral at 5.2%. This REALLY limits train length. A Shay can still manage 8 cars but locos in the class of a Big Hauler can handle 3 cars at best.
It can be done, just be prepared for the consequences.
I have a 3.4% grade on my mainline, along the back fence. The approaches are all level, and the grade is on the straight track. I need more locos to pull long trains, but MU operation and doubleheading is fun.
To complete the mainline, I have a big long 40' section of downhill that is 5.5%. It's steep, but works well. I can run short trains up it also, like 10 cars.
Nasty grades, but workable. I think I saved myself by not having any of the grade on the curve.