Cork would be a good option to raise and level the areas between tracks, but another option might be Durabond patching plaster.
My turntable tracks were all laid directly on the plywood of the layout's structure, which allowed level access into the roundhouse....no need to jack it up to get everything on the same level.I use Durabond 90 for all of my scenery, applying it over aluminum screening, supported by risers where necessary.
For your situation, you could simply mix up a batch, dump it onto the areas which needs to be raised, then level it out, more-or-less, using a putty knife or scraper.
I use Durabond 90 (the 90 represents the number of minutes in which it will harden), but there are other versions with shorter setting times.
It will harden as indicated by the number, and will not crack, probably even if you were to jump on it. I also use it for casting bridge abutments and piers, using homemade moulds.
I used Woodland Scenics fine Cinders for ground cover at both of my turntables, but later made a couple applications of unsanded black tile grout, as even the fine WS cinders otherwise looked too large.
Here's the turntable and engine shop at Lowbanks...
...and the roundhouse at Mount Forest...
I also used Durabond 90 to create all of the "water features" on my layout. All were applied on pre-wetted plywood (just a sprayed application to prevent the plywood from sucking the water from the Durabond), and most of it is only around 1/8" deep...no cracks in over 30 years of service, and no scuffing despite dozens of cameras placed directly on it to take photos.
A couple of pictures...
Because you're working in N scale, you might get better-looking results if you ballasted with only a very thin layer of WS fine cinders, and once they're affixed in place, spread unsanded black grout over that entire area, then mist it with "wet" water to affix it in place.
Wayne
I used sheet cork in my yard. You may be beyond the stage where you would be willing to redo the area. I suppose you could still cut up sheet cork to fill in between the cork roadbed.
I suppose you could use sand, which would be a pain to level, or pour a runny mix of plaster. Drywall compound tends to crack, but since it's going to get covered in ballast, maybe it's not such a bad choice. Cover your tracks in painters tape so you don't have to deal with stuff getting on the ties.
Henry
COB Potomac & Northern
Shenandoah Valley
I have am modeling a free lance version of the Age of Steam roundhouse. I am getting ready to ballast. In pictures it seem the balast around the reounhouse is fairly level all arround. My tracks ar laid on cork. what can I use to fill inbetween the tracks so I am not juts dumping a lot of balast to make it a fairly level grade
Thanks
Perry