I used 1/4 inch plywood under my track rather than cork. I like the sound of the wheels going over the track, I think it sounds more like a real train.
To transition, I poured sifted dirt between the ties and soaked it down with the glue/water mixture or scenic cement (Woodland scenics product) which made it hard and non dusty. Then either ballasted it or left it like a dirt roadway.
This is kind of unconventional, but it worked well and if you check out this video I show how I did it:
[video=youtube;dgkNpazxDZI]https://www.youtube.com/watch?v=dgkNpazxDZI&index=5&list=PLmDLuMmpivK4qL5eq3iyErQUVP25Dv2hK[/video]
I kind of sped by the transitions but at 30 seconds it shows the two types of track together and also at about 1 minute, there are more videos the ones numbered 1 through 8 show the whole layout construction.