I would bet the farm that they have been replaced, a number of times over the years. They don't make it a habit of doing that type of repair while Guests are watching. Common sense would dictate that after all these years, everything other then the outside shell has been replaced, the undercarriage and drive systems just wouldn't last this long and still be the original equipment. The carpets would be threadbare and the AC are like every AC in the world... they break down at the most inopportune moment. Again though after 45 visits over 35 years, I have never been in a monorail cabin when the AC wasn't working. Stuff happens. When I drove bus, I would start the day with AC that would freeze meat and within a couple of hours something would break be it compressors or circulation fans. Not often, but, they did and they were working perfectly when put on the road. Somethings cannot be controlled even by pixie dust.