This question came up in this thread – http://boards.straightdope.com/sdmb/showthread.php?t=351535 – but I never got a clear answer to it, so I thought I’d pose it in a thread of narrower focus.
http://en.wikipedia.org/wiki/Light_rail:
http://en.wikipedia.org/wiki/Monorail:
What are the relative advantages/disadvantages of each? What should guide a transit planner’s decision to choose one or the other?
Light rail has a couple of advantagers over a monorail in that they are generally cheaper to implement, and that monorails, being elevated, tend to be considered eyesores. I don’t agree with that opinion, and marvel at many of the things around me that are not considered eyesores, like billboards, store signage, minimalls, fast-food joints and so on.
A disadvantage of light rail is that in many sections it runs on the street, like an old fashioned streetcar, and can get bogged down in traffic like a bus.
I have also heard from people who should know that monorail switches are complex and expensive. Standard train switches are simple, reliable, and inexpensive. According to my source, the lack of efficient switching is the main reason monorails never became very popular.
By gum, the monorail put Brockway, Ogdenville and North Haverbrook on the map!
I’ve noticed that many monorails are constructed in places where it would be impractical or impossible to procure the necessary land for light rail - namely, through already populated areas. The Tama Monorail, Shonan Monorail and the Okinawa monorail, for example. Also they seem to allow for sharper curves and steeper grades than light rail.
A problem with monorail that we ran into in Seattle recently was difficulty evacuating people during a fire. Because it’s up in the air and there is no platform to escape to, fire trucks had to bring people down by ladder, which was slow.
Isn’t one reason that monorails have taken off in Japan that the beamways are standardised there, leading to lower costs and more equipment choices?
The Las Vegas Monorail is proving to quite successful. After they fixed it, that is. 
That was a bit of a gamble, wasn’t it? 
Is there any reason you couldn’t elevate a light rail track? If that’s the only advantage to a monorail, why make it mono?
Light rail tracks often are elevated where they have their own right of way.
And having their own right of way is a very good thing. Ask anyone who lives in Houston. When the light rail was implemented last year it averaged TWO HITS A WEEK for a few months! (And that’s not counting the deaf guy who walked into teh side of the train or the lady who was trying to commit suicide.)
http://www.briannelsonconsulting.com/houston-metro-rail/accidents.html
Anyone who has seen both the Seattle monorail and the Vancouver skytrain should realize that there is no difference in visual impact between a monorail and light rail. In fact if you look at the pictures found at http://www.elevator-world.com/magazine/archive01/9712-001.html-ssi of the oldest light rail system in the world, you will see that the visual impact of monorail can be quite large. Other advantages of light rail are that it is equally happy with elevated tracks, subway tracks and street level tracks whether separated from the road or not. The technology is well understand and standardized. Equipment can be bought off the shelf and the switching system is much simpler. You simply get much more bang for the buck and I can see no advantage whatever for monorail.
