This should probably be linked to somewhere in the first post
http://www.jonnyguru.com/forums/showthread.php?t=1036
Micky_lund you may want to take a look at this thread about single vs multi rail PSUs, but i would really suggest reading through the entire PSU FAQ its very informative.
http://www.jonnyguru.com/forums/showthread.php?t=3990
What those current ratings actually mean is the point at which the PSU will trip and shut the rail down, continuing with the picture that Proximon posted, you can draw up to 18 amps from each rail, if you try to draw 18.5 it will shut down to protect itself, however the 12 volt source itself cannot put out more than 80 amps of power, or about 13.3 amps per rail if the load is evenly distributed, so even though you could draw 17 amps per rail without triping any individual rail, the source for those rails would be over taxed and shut the unit down as a whole.
The reason you cannot just add the rails is because they have a common source, if they each had an individual 12 volt source that was powered off the AC in then yes you could add them, but the added hardware to do that would make PSUs obscenely expensive and horribly inefficiencient, there are some good PSU that you can just add the rails on, certain Antecs and OCZs, but thats only because they rate theirs honestly.
In the end the number of rails a PSU has is totally irrelevant since its really the combined power we care about in the end. Even intel changed the ATX specs from requiring the CPU on a second rail to recommending it so they atleast have come to believe that separating your rails doesnt provide you added benefits unless the PSU is crap to begin with.