I want to elevate 1 line about 7" high and 16' long what do I use 1/2" plywood or can I use 1/4" underlayment nothing will be seen thanks Roy
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A lot of folks don't think 1/2" is strong enough over the long haul, so I can't imagine anyone suggesting 1/4". I would think 1/4" would have too much give between risers. Most use joists that are 16" apart to attach the risers and I'm sure you can imagine how much 1/4" will sag over that span. You can add more joists/risers, but that just creates that much more work and 1/4" will still sag over even an 8" span. I think 3/4" is overkill, but then I don't intend to run long trains with heavy locomotives.
EDIT: This assumes you are using open framework with risers on 16" center, etc. If you have a tabletop type layout where you can add support along the entire breath of the grade, you don't technically even need roadbed, you can put track directly on trestle supports.
This elevated track is secured to 3/8" plywood supported about every 12". That's adequate support for a Premier Big Boy with no sagging.
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I've seen others use 3/8" with a trestle set as well, Susan. I've used a trestle set with no roadbed at all, so I have no doubt 1/4" would work for that. I wasn't thinking in terms of a trestle set. 
This grade work was done with 1/4 luan, and cork, supported as needed. Pictured 8" on center support. The 1/4 worked well around the corners, bending in 2 directions.
If your elevation is nice and straight for the entire length (or for substantial length sections), consider using a couple pieces of 1/2" angle iron. You can easily span 3-4 ft with virtually zero sag.
Set the two pieces facing each other - and you have a 'trough' for the track. Add a couple of well placed cross braces to maintain spacing - and its done. You may also want to grind off the ends to a smooth curvature. Prime. Paint...or leave au natural.
For shorter runs, you can get aluminum to keep things light.
