A Mean Well multi rail power supply is just a single rail source split up into either two, three, or four output voltages.  This section of the power supply does not actually provide the power, it carries the electricity itself through tiny copper pathways on a motherboard.  So what is the reason for splitting the source?

The biggest reason that single rails are split into multiple ones is safety.  The best comparison is a common circuit breaker in a home.  There are different switches for different areas of the house, one for the kitchen, one for the basement, and so on.  This is so that the currents rushing through the wires can be limited.  If the currents are too high, the wires will get dangerously hot or shorts will occur more frequently.  Forcing the currents to travel down multiple wires makes the voltage cleaner and provides stability.  This is how a multi rail works on a motherboard.

If the current going through an output increases past a certain point, the power supply will completely shut down.  Multi rails allow for multiple traces of power to run through outputs and single rails allow for one.  The only difference is how much power is allowed to run through each output, which can vary.

Simply put, multi rail units shut down when too much energy is run through the output, which affords protection for the technology.  A single rail might overheat to the point of destroying the equipment.

Mean Well multi rails come in dual, triple, and quad output voltages and range from 35 to 375 Watts in their selection, and are guaranteed to have safe and high-quality performance.