If you're good with a table saw, you can make spline roadbed -- a series of strips of material laminated to form a thick roadbed. Each spline can be separated by space pieces. Here's a link that gives a good description: http://s145079212.onlinehome.us/rr/howto/splines/
Curves have natural easements when spline roadbed is used, plus creating turnout splines is very easy -- split the spline for the diverging and non-diverging routes, fill the gap and keep going. The spline doesn't have to be solid. You can put a spacer between each spline and top the roadbed with Homasote. It will be quiet, will support the track well, and when creating a grade the transition from level to grade will be smooth (you can't make sharp vertical bends).
Another option is "Ladder" roadbed. This is what I refer to as a spline variant but it has the same positives as spline with a slightly easier (albeit tedious) construction. You can make ladder roadbed using PVC from Lowe's (see Ron Karlsson's video here: https://www.youtube.com/watch?v=NvXzN2vz2OU). It's available in 1.25"x1/4" strips (sides) and in "1x4" (actually 3/4"x3 1/2") which you can chop into 3/4" square stock to make the "rungs". The build is a bit tedious but you get an open, quiet, well supporting road bed. For more information you can go here: http://familygardentrains.com/.../roadbed/ladder1.htm
Note that they used HDPE, which is sometimes hard to get. PVC works just as well. The article is for G Gauge, but can be done with O gauge (our viaduct on the club layout is done with HDPE ladder roadbed to which we glued bridge girders.)
