gmcrail:
The Chicago commuter lines have both the smooth-side and corrugated cars. The cars tend to be run on the lines where they originated - smooth-side on the former C&NW lines, the corrugated on the former Burlington and Milwaukee Road lines. They are still using the original equipment, which has held up quite well over the years; they've been repainted, of course. I had the opportunity to ride the Burlington-line Metra trains recently, and they ride just as smoothly as they have always done. Personally, I like the corrugated cars more than the smooth-sided, but that's probably because i grew up watching the Burlington's trains roar down that 3-track western main line.
It is my understanding that since the passage of the Americans with Disabilities Act, and the requirement of ditch lights, Metra has been replacing the original bi-level equipment with new cars. The new cars are all corrugated stainless steel. I can't remember the last time I saw a smooth-side commuter car on the Chicago commuter lines near me.
I overheard a conversation once in which someone said that the old bi-level cab cars - the cars that have a cab for the train crew - cannot be fitted with ditch lights. So these old cars now have their cabs locked up, and that only the newer cars that were built with ditch lights are on the point.
IIRC, the Con-Cor commuter cars mentioned by Mr.LMD represent later production (I want to saw very late 1950s - 1960s), while the Wheels of Time cars represent earlier cars (mid to late 1950s), some of which were used on intercity passenger trains.
Mr.LMD, you might want to look at this website. Notes on the Bi-levels can be found here (smooth-side), and here (corrugated, stainless steel).
