In the past, I've used HO scale Midwest Cork roadbed for the mainlines and N-scale Midwest Cork roadbed for sidings and branchlines. I simply extended the HO cork about six to eight inches into the siding, then sanded a gradual transition using a home made sanding block with 220 grit sandpaper stapled to it. Worked really quickly and the transitions don't have to be very gradual as they're pretty steep prototypically.
I haven't had a lot of luck with WS foam roadbed and I don't recommend it, although I realize a lot of people use it. I much prefer cork, simply because it can be easily sanded and modified.
Nowadays, I'm just sanding down N-scale Midwest cork on the sidings using that same old home made sanding block, then laying ME code 40 for the Medium Duty trackage on the sidings.
If you're interested in representing medium duty trackage using only code 55, you can cut the spacers between the ties and move the ties apart a bit. Medium duty track's ties are almost always spaced further apart than heavily trafficked mainline track's ties, even on sidings used regularly for mainline trains to pass, such as on UP's center sidings in Weber and Echo Canyons in Utah which obviously have lighter rail as well as less densely spaced ties.
Of course, I'm talking about trackage with wooden ties. Concrete ties are out of my era and I know virtually nothing about them, other than they're gray and ballast sits on top of 'em in the middle.
