I cut an incline out of foam and can concur that it is both messy and tedious to get a nice smooth incline. It can be done, but there are easier ways. I have also used the method of plywood cutout but it also needs work at the top end to ensure a smooth transition.
Foam cut:
Plywood:
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There are lots of different systems. The best one for you probably depends on how your benchwork is constructed, and how much effort you want to put into the project. The Woodland Scenics foam risers you referred to will give you a well-defined grade that you can curve easily, but you'll pay for the ease of construction. At the other end of the scale, using open benchwork with spline sub-roadbed will produce a fine result, but the initial effort and learning curve can be daunting.
I have a simple, open frame bench, and I use 2-inch construction foam for my base. I use supports made from 1x2 lumber to elevate sections of the base to get higher elevations. Where I need to change the elevation of the track, I simply cut a piece of the 2-inch foam and use it for a ramp, supporting it at varying elevations to get it to climb. It looks like this from underneath:
I tried cutting a ramp out of foam. I had a lot of trouble getting the nice, flat contour I wanted. This technique of using a flat piece of foam and varying the supports works much better.
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