Plastic decks are made from 100% plastic. In most cases it is reclaimed plastic dyed a color. The deck support frame is wolmanized wood and just the top boards and rails are the plastic. Some of the plastic have added grooves to add traction and reduce the sun's heat build up. The cost can be as much as 100% more than wolmanized wood. The pictures below show white plastic handrail systems but you can buy plastic 2x6, 2x4, and 2x2 components to build standard handrails.
    Another option you have is to keep your treated wood deck tops and just replace the handrails!