What is the difference between the gray roadbed and black roadbed attached to the track?
The Grey Road bed is Nickel Silver Rail which is preferred because it is corrosion resistant looks more real and its oxide is electrically conductive. The black roadbed is steel which can rust over time. Also the grey roadbed has black ties that while not perfect do look more real, if you do not plan to ballast your track.
Having said that I have used both without problems but having replaced the steel with NS I have found the NS has less electrical resistance leading to better performance overall.