I often ramp down from "HO" to "N" cork so passing tracks and industry spurs are lower. In some cases I've also omitted roadbed under spurs entirely, installing track directly on the subroadbed for an even greater difference in height.
Here I used thinner cork for the industry track, along with smaller rail (code 55 vs. code 83 on the main). Note also that weathering and ballast make a big difference in how the different tracks look.
From left to right in this photo of the BNSF in Lamar, CO are industry tracks, a passing siding, and the main. Note again the differences in color, ballast texture, and sizes of rail, along with the changes in height.
This area has three different track levels, on HO cork, N scale cork, and none at all unde the Hudson Press spur. The ramp leading down from the HO roadbed to the spur is visible at left. I tapered it with a belt sander. Codes 83, 70 and 55 track were used.
The same locale looked like this once it was completed. The extra work is noticeable in the finished product, and visitors often comment on it.
Rob Spangler
I use the rolls of 1/8"cork sheet from Hobby Lobby for the track roadbed of sidings and yards. I run the mainline roadbed (Midwest Cork) up to it and make the transition with a Sureform tool. I save the resulting cork dust for landscaping and fill material.
Jim
My main line is on cork. All other tracks are on the layout surface.
Sidings range from, 'used every couple of hours' with spurs to all the adjacent buildings in the industrial/commercial park, to 'scruffy remains next to a business that now ships by truck.' Obviously, two very different 'looks.'
My suggestion is to hunt up some local examples and go look at them. A couple of hours on the public roads adjacent to the tracks should result in a camera-load of inspiration. Supplement that with notes on rail height, ballast (or lack thereof, and what substitutes for it) ground cover... The prototype is always the best guide.
As for how high roadbed can be stacked - here in Northeast Clark County, the UP (LA&SL) runs on a couple of feet of grey rock ballast on top of a yard-high earth embankment. This is an area where Mother Nature has been known to deliver a year's 'average' rainfall in one twenty-minute feline and canine downpour.
Chuck (Modeling Central Japan in September, 1964 - to my own field notes and photos)
I use the Homasote roadbed from Cascade Rail Supply. They have 18" and 12" scale thickness and transition pieces available. I use the 18" for mainline, 12" for passing sidings, and industrial tracks are laid on the ground.
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What they said!
Rio Grande. The Action Road - Focus 1977-1983
As a rule sidings are at a lower level than the main (in part to prevent cars from rolling from siding to main). So slightly lower siding, which can be done with using N scale sized roadbed versus the main. Also, smaller rail. Minimal to no ballast. Weathering the sides of rail more on a siding; more vegetation on the track and between ties. Wider tie spacing which can be done with flex track by removing a tie or two every six inches and then spreading the remaining ties out.
Not often modeled but sidings with sharp curves often feature gauge rods which hold the rails together where they otherwise might be spread apart.
Dave Nelson
Use a smaller code rail on the sidings along with the N scale cork.
Roger
I would use N scale roadbed or no roadbed at all for the sidings.
I am probably moving within a year, which means a chance to build a new layout. I think I would like to work on my track work. I would like my sidings and spurs to look less used then the main line. I have read about a few techniques on how to model this, one of them uses double layer of cork on the main. That seems to put the track high off of the table. What suggestions do you have?