I am new to 009 modelling and would like to produce a layout with several level changes. What severity of gradient is possible and /or sensible for fault free running?
Unfortunately I don't think there is a universal answer to that question. The answer depends on so many factors, including length and weight of the train, the severity of curves and probably many other things.
I suggest that perhaps the most reliable approach is to carry out some tests on a mock-up using your locomotives and stock.
For a given train of a certain length the gradient along a straight run can probably be greater than for a tight curve and the situation worsening with increasing angle of the curve, so it is probably a good idea not to do a test on a straight run and then apply that value to anything but the gentlest curve.
It does all depend on what locos and rolling stock you intend to use as well as the length of the trains.
However, I have a rabbit warren type layout, built by the lat Jack Carter with 1 in 25 grades. This does limit the weight of the trains and also each loco has to be fitted with as much lead weight as possible. This layout also has 9-inch radius curves. The combination does significantly limit the locos and rolling stock I can use on this layout.
Here is a photo of Meanderbahn:
(https://009adventure.files.wordpress.com/2018/09/meanderbahn-brian-meldon-1.jpg)
Originating web page: https://009adventure.blog/2018/09/07/beccles-exhibition-2018/
My other layouts have a max of 1 in 30 grades and minimum of 12-inch curves this is a lot easier and less restrictive.
You will also find that adhesion can be severely affected by what you put on the rails and what you clean them with. Railzip and carbon-based products will make the locos slip as will cleaning the rails with any type of cleaning fluid. Using a fine track rubber is best for adhesion but can make the rails get dirty again quicker as it will introduce tiny scratches on the rails.
I don't know if it's really any guide but my "High Water" diorama has the following :-
1. Total track length ~ 5 feet (Peco 'N' gauge)
2. Approx. height difference (end-to-end) ~ 2-4 ins.
3. Curve radii (2 of and, opposing) ~ 6-8 ins
There is a goods siding in the station area which is almost flat but still has some small gradient. Loco's often stall here ! (Something of a mystery...)
The steepest rise is from a tunnel exit up to the goods siding. Taking the body off my Kato chassis'd diesel shunter leads to wheel slip on this section (there are two car wheel type weights under the cab roof).
The track is cleaned with a proprietary cleaner ('Slaters' I think), or, IPA before use.
A 'Grafar' shunter chassis (0-6-0, Plastic Body) also climbs this slope Ok but, has the advantage of bigger driving wheels compared to the smaller ones on the Kato.
Hope this helps,
Dave
ps Let me know if you need more precise measurements etc. (I'm doing this from memory !)...
I may pushed the envelope (below) but it's designed only for 1 short carriage or 3 vans / wagons!
Giles