Frank B says:

Basic thing to remember, is railroads don’t like gradients (prototype or model). Include gradients on your layout for realism and interest, but test the slope with the loco and the heaviest train it will pull. 
 
Or if you want to get technical, weigh the cars, and check the locomotive’s traction with a small force gauge or by hanging small weights over the end of a track and see what it will lift. Then use the formulas in your school physics textbook for inclined planes. Adding weight to a locomotive will increase its traction.

https://en.wikipedia.org/wiki/Track_geometry#Longitudinal_elevation 
https://en.wikipedia.org/wiki/Grade_(slope)#Railways

Also the calculator page mentioned above has some useful additional information.