I make my asphalt roads from Durhams Water Putty, a hardware store product.

I build a ramp up to the track level, and put the putty on top of that, just to the top of the railheads on either side. For the part between the rails, I cut a strip of thin styrene and shim it up so it's level with the track. I use cheap gray acrylic paint for the putty road and the styrene center. I use a bit of weathering powder between the rails, too.