I removed the corkboard on the yard and decided to put the track directly on the foam. It seems many master modellers do that and even suggest having different heights between mainline and sidings/yards. Too bad none explain how to easily transition between mainline and yard.
Other posters have asked this question, but the frequently suggested idea–thin cardboard unerneath the track–didn’t stop the cars from rolling downwards when I did that. How do I prevent the cars from sliding downward when they travel from mainline to yard? Quite annoying having spent most of the evening working on this effort!
I have used this slope to my advantage to simulate a hump yard. This works especially well if you are using kadee delayed action couplers, and have a flat spot on the yard lead to use for the uncoupling magnet. You do need to have a slight elevation on the ladder track so that cars will roll through the switches. I initally discovered how to do this when my train board sagged in the center of my yard.
On my HO layout the yard is intentionally lower than the ladder or main to prevent cars from rolling onto the ladder or main. The transition was simply allowing a piece of flex track to assume its natural curvature from the last turnout to the lower elevation in about 8 to 10 inches. The transition track was secured in place by allowing dry ballast to fill the voids. It was then wet down with 50:50 alcohol and water with an eye dropper followed with 50:50 white glue and water in the same manner. If you make a mistake no problem, simply wet down with the alcohol mix and it will lift right up. A light brushing under running water will return the track to like new.
To prevent cars from rolling on moderate slopes, insert vertical broom straws in appropriate holes drilled in the roadbed in convienent locations and sticking up just high enough to foul the axels.