I was watching Fahrenheit 451 yesterday, and can’t help wondering exactly where the outdoor shots were taken. The monorail on which the main character commutes to work looks real, but seems somewhat futuristic for 1966. Also, I find it hard to believe that ordinary citizens were expected to climb up and down the steep ladder which got lowered at stops.
Was the monorail left over from some World’s Fair or similar event?
IMDB sez that the monorail scenes were filmed at Chateauneuf, Paris, France. The interiors were shot at Pinewood Studios in the UK.
Yeah, in one scene it can be seen going backwards and then forwards again to simulate the prescence of “traffic” on this line after dropping off the main characters 
Makes me wonder what the standing passengers in the aisle were supposed to do when the floor dropped out to become the ladder. They probably had to hang from the handstraps by their hands! 
Expanding on Gobear’s answer:
It was the SAFEGE monorail, built in 1958 in the area south of Paris known as “La Beauce”. The test track (which was never used for passenger service) ran for a few kilometers alongside the main SNCF Paris-Orleans line, near the town of Chevilly about 15km north of Orleans. (I used to live in Orleans, and took the train to Paris regularly, so saw the no-longer-used monorail track on hundreds of occasions). I’m not quite sure why many web sites, including imdb.com, refer to the monorail as being at Chateauneuf-sur-Loire, when that town is much smaller and further away from the track than Orleans is.
monorails.org has a photo and a short history.
There’s also further information here and here from an American named Randy Lambertus who went hunting down the relics of the SAFEGE monorail in 1998. The latter link has a photo of the monorail car with the “emergency escape staircase” deployed. When the train was in normal service, passengers would have exited through the regular doors onto the station platform at car level. The staircase would only have been used in an emergency evacuation when not in a station (similar to an aircraft’s emergency escape chutes after a crash landing).
After following the links in my own post above, it turns out that there were two experimental monorail systems built near Orleans:
The SAFEGE monorail, seen in Fahrenheit 451, with the car suspended from the track. The test track was near Chateauneuf-sur-Loire, only 1400m (less than 1 mile) long, and no longer exists.
The Bertin Aerotrain, with the car riding on top of the monorail. The test track was 18.5 km (over 10 miles), had three stations, and still exists (as of 1998 at least). This is the one that can be seen from Paris-Orleans trains near Chevilly. Photos are here.
Mea culpa.