There are three or four catches molded into the clear plastic windows on each side. These have to be pushed in slightly (and/or the top of the window where they catch bent out slightly), and a thumbnail or similar size wedge gently worked into the area just above the catch between the carriage side and the roof. When all are released, the top comes off easily, but BE CAREFUL! If the coaches are getting old, the plastic is also getting brittle and will scratch or crack.
ps When you get it apart, file the catches down so they are almost flat. They don't need to be anything like as large as they are, they are way too obvious, and it will make it a lot easier to get the carriage apart next time.