If you get a true multiple rail they are actually better, because the rails are separately regulated and therefore isolated from each other, but they are very very few and far between. 99% that are called multirail are really pseudo multirail, because the "rails" are only separated by a current limiter, much like a breaker in your house. That's the 240VA or 20amp limit.
The reason for the pseudo multirail is for fire safety. The problem is that the current of each rail can only be a maximum of 20A, so if the rail is under loaded the additional current is unusable. Since the video cards started drawing a lot of current we got the problem of rail balancing to ensure that the video card plus the other components on that rail didn't exceed the current limiter (breaker) on the rail. So many PSU manufacturers raised the current limit on the rails. Or they only put one really high current limiter (breaker) on the whole PSU, which made it a single rail.
Rather than give you an example I'll just post the one from
PC P&C.
Quote:
8. ARE MULTIPLE 12-VOLT RAILS BETTER THAN A SINGLE 12-VOLT RAIL?
With all the hype about multiple 12-volt rails (ads claim that two rails is better than one, five is better than four, etc.), you'd think it was a better design. Unfortunately, it's not!
Here are the facts: A large, single 12-volt rail (without a 240VA limit) can transfer 100% of the 12-volt output from the PSU to the computer, while a multi-rail 12-volt design has distribution losses of up to 30% of the power supply's rating. Those losses occur because power literally gets "trapped" on under-utilized rails. For example, if the 12-volt rail that powers the CPU is rated for 17 amps and the CPU only uses 7A, the remaining 10A is unusable, since it is isolated from the rest of the system.
Since the maximum current from any one 12-volt rail of a multiple-rail PSU is limited to 20 amps (240VA / 12 volts = 20 amps), PCs with high-performance components that draw over 20 amps from the same rail are subject to over-current shutdowns. With power requirements for multiple processors and graphics cards continuing to grow, the multiple-rail design, with its 240VA limit per rail, is basically obsolete.
PC Power and Cooling is once again leading the industry. All of our power supplies now feature a large, single 12-volt rail. The design is favored by major processor and graphics companies, complies with EPS12V specs (the 240VA limit is not a requirement) and is approved by all major safety agencies such as UL and TUV.3
A single rail provides the total current to all connectors. The amount drawn by the connector is subtracted from the total and the remainder is fully available to all the other connectors.
The problem with single rail is that if there is a short that doesn't draw more than the PSU current limiter, which is much higher than 20A, then the PSU won't trip off but will continue to deliver power. That current is high enough to melt wires etc. and start a fire, so you have to be more careful when building.