Ntrak specs call for the sub road bed to be 3-1/8 inches higher than the main lines. I cut a strip of 1/2 inch plywood 2-7/8 inches wide, then drilled a series of holes with the hole saw. I was able to make these holes with the drill press which made it easier. This got attached edgewise on the line 4 inches from the rear of the module. The photo above shows this from the rear of the module.
For the actual sub road bed, another strip of the 1/2 inch plywood was attached to the edgewise to give the mountain division line the required height of 3-1/8 inches. As there would be a siding for the station on the front side of the mountain line, several scraps 2-7/8 inches long were placed as shown in the photo above to support it. Besides carpenters wood glue the edgewise strip was attached to the module with screws and the sub road bed was attached to edgewise strip with 7/8 inch finishing nails from a nail gun.