Transition from cork roadbed to sub-roadbed

What material can I use to get the grade level from cork roadbed down to the sub-roadbed?

This transition is for the two service tracks that will lead to an engine service facility and I want them right on the table top.

I tried to fit a shim used for hanging pre-hung doors but it was too thick to start with by about 1/16th. The slope because of the length of the shim would have been perfect to use but it won't fit under the track at roadbed level.

Any ideas will be considered. Thanks.