What are PSU RAILS, what are Virtual Rails?           

I will lead off with a short section on safety, fire safety is my worry never being electrocuted, (have GFI here)  DO NOT READ THIS and jump to what is a rail.
The Best PSU made offer, FULL PROTECTIONS @" OCP, SCP, OVP, UVP, OPP"  (the last letter P means protection )
Over current , Short Circuit protection, over and under voltage protection and last OPP  means over power protection)
Ponder for a moment?, a fault that allows   5vdc going to 15vdc.  (BOOM) YOU LET THE SMOKE OUT $1000 worth of chips ! OVP is the cure. (All most all PSU made for 20 years have this feature)
The purpose of the PSU is to provide perfectly 
Regulated and clean (means no noise) DC power, and to not blow up $1000+ worth of  hardware in the PC. (CPU/RAM/MOBO,GPU card, SSD, etc)
The second purpose was not to run 20 amps up to  140 amp.'s of current onto say one 12vdc 18gauge yellow wire, and now you have  a PC ON FIRE.   (so the new rule is burn proof wire, and sparks and smoke is now ok at the  UL)
Starting in 2020 year, (or just before then,"NOW") They changed the rules on safety (not human shock or electrocution reg.s , but fire safety )
(per UL 1950/CSA 950 / EN 60950/IEC 950, reg.'s v2.3 ATX removed the 2.2 spec limit , is gone in v2.3+
Most PSU sold today are safe from fire , with 240VA limited old spec.  but this is no longer true.
The old stardand will be gone in year 2020, with NEW 62368-1 (all new and very different) but
. "After considerable study by the Technical Committee, the energy hazard requirement has been removed from IEC 62368-1".<  here are yor culprits.
My wild guess is the BIT COIN MINING ,GOONS caused all this. (ATX spec., not changing wire guage and connectors  is too hard for them so they decided to let the bugger burn)
If you buy 1800w PSU what will be the max 12vdc amperage (OCP) the answer is 140 Amps.  That is what the big PSU does, and is the worst case example , in fact all over 20amps are bad.(ATX spec wire and connectors)
Imagine a single 140amp 12vdc rail with many 18AWG wires leaving that point , like an  octopus. (and then think of single point short failures) I own 3 welders (TIG/MIG/STICK) so I really know what a short looks like !!!
The new cure for this is (do nothing much in the PSU but on the outside new VW1 rated wires)
Proof: (the new regulation on safety)
Clause 6 of IEC 62368-1 requires safeguards to be in place against fire under normal, abnormal and single fault conditions. (the 21st Century cure is this, wire that will not burn ! but exploding , well that is ok to them.)
Took me a while to understand they offer 2 cures, to stop the fire from 18AWG wire burning and starting a fire,  (use burn proof wire(teflon pops to mind) or  Control of Fire Spread.(the PSU make did not make the PC so how does this work?)
This is silly in that you can put a new PSU in any of 10,000 makers of PC, ATX spec PSU, and all  wiring classes and distances are totally unknown.  (here is the only place talking about this good ole' UL)
The truth is , if one thing  in any PC shorts, and is on a single 18AWG (American wire gauge) wire as all PCs have (some are 16AWG , rare) then that wire explodes, and shoots spark to place not  good, I have seen a PSU do that and the fan in the PSU blew sparks to the floor rear for 5 full seconds, (see it and know why I do)
Testing for fire safety is a tricky set of tests, not only power on rail shorts but fast acting or slow action rail over current conditions, (and then a fire starts on say 1 wire) (a the least 3 tests must be done for OCP compliance per rail)
  1. Power on a rail is 0 ohm short. (may be safe, who knows, do the test silly)
  2. Power was already on and 0 ohm short happened, (the DSP class supply may catch this and drop off line in 16mS or less. (you may get just bit of smoke and stink)
  3. Same as #2 but we sneak up a 90% of full load, say 90% of 140 amps or 126amps and watch that ONE 18AWG burn like 4th of July Sparkler.  smoke and sparks many, and the only FUSE here is the bloody wire on fire.
Most of these PSU and PC run cheap easy to burn 18AWG wire and the molex pins are rated at 9 Amps max.

140amps flowing on any 18AWG wire will burn  fast and mighty and throw lots of white hot burning sparks and smoke,  you can take that to the bank.  They lamely only offer 1 cure.  UL 758 spec wire., or UL 2556 VW-1
18AWG is ISO size 1.02mm or 0,823 mm˛ (Yankee to European standards) YWG is better name LOL.

Try to learn that any 1 wire can in fact short. (example the HDD/SSD/DVD drive shorts that 1 yellow wire to ground) then what?, do you like a fire?.
The Rails are either REAL or Virtual,  (that is correct there are 2 kinds of PSU not just total power and amps per rail. ok?)
The word Vitrual here means a current limiter regulator on each small rail 20amp (typ) as seen below clear as day. (schematic)
With the new laws, virtual  rails will go away forever. no need for real OCP. Not now. (sarc off)

What is A Rail?
The old practice for 97 years now, we built prototype types on  a thing called a "Breadboard" with copper power bus bars on top and bottom ground  and wired circuits up to between them for prototyping and then made a drawing.
The schematics drawn up for vast years were the same way, ground on bottom and power on top. (vast examples exist,100 years for sure, and many a ham long ago used wifes breadboard to make  transmitter or something not sold in stores)
If you look at your breadboard, power buss drawings,  it looks like  RAIL road tracks, and that is how the word RAILS began, in a LAB making new designs in electrics and electronics later.
The word  "RAILS means" ,really  POWER RAILS. (2 words) If you forget ground then the below has 4 RAILS. (real)
The first schematic below photo 1,  is of very good and very rare and costly  ,  dual  +12vdc rail PSU.  ( photo 2 is a real and common virtual rail PSU)
See the Transformer on the top left below?, and 2 full wave diode pairs, and that output coil-cap pair (that  inductor (LC) coil keeps the chopper noise off the DC line) called also noise filter. (LC filter)
That pair of diodes form a  REAL RAIL source.  (I will only talk  about 12vdc rails, as that is our only topic here)
You can see there are 2 , 12vdc rails below, so it is  a prime grade PSU, but a cheaper  PSU called  Virtual RAILS has only 1 huge 40 amp RAIL and a Y  fork to 2 , RS1 and RS2 resistors. sensing. and to the 2 virtual rails out.
The the maker puts 2  RS1/RST 0.1ohm resistors at that  fork and then dynamically limits current to 20 Amp.'s each (240watts) each, and is a Virtual RAIL.
A real PSU with 2 real Rails makes less noise and has way better transient response times.

The below PSU is real RAIL, not Virtual. (the VS pins are voltage sense pins and the "IS" pins are current sense, see?)
The 2end 12vdc rail transformer is not shown but is the same. denoted as +12v(˛)Coil. and same for coil (1) 12v. (The Rsx (0.1ohms) below are what forms the OCP features, over current)
Photo 1:below (a REAL non Virtual RAIL PSU)! RARE and expensive the transformer sure is more costly) Weltrend is another top brand chip that can do this.
This is a chip called. "Silicon Touch Technology" or STT, and is 2008 designed, the PS223 or PS228 with 6x 12v output monitors.
 The below was a great design, but  using $1 chips is too costly  it seems (sarc off).  But really is a lie, do to this fact: Today a DSP emulates all that and more.(and is not costly at ALL)

Do not open any PSU case until you attain , serious HV training, working with High voltage or risked being dead (it is LETHAL, not a joke, IS ) Not the low voltage side the otherside of the chopper.
If you open up your PSU Case , most will have all the yellow wires soldered  to a SINGLE PCB trace PAD and for sure a 1 RAIL PSU 12vdc,  if that rail is 62  AMPS RATED , AND YOU Short out 1wire say with a bad HDD , you will get fire.
The rails vary from 20 amps max to to over 125amps (1600watt) for BIT COIN FOOLS mining.
The above PSU will trip at 240 watts, or 20 amps or near( I did the math).  New PSU can do 50 amps  easy, and can catch fire with what is called  single fault. (a.k.a. a shorted rail)
The newer the PSU the smarter that chip is above. (some made 2010 and newer are vastly superior to the older ones , in fact even allowing users controls on OCP (software USB applications)
If say a shorted HDD shorts and say 0.3 ohms, not 0.000 ohms, dig?, then  the HDD short burns with the wiring up . (in fact 2 fires burning (yellow and black) and super toxic smoke too.) 
The PSU has really 3 goals, run the PC with correct clean voltages, not electrocute you and to not burn you house to the ground. (3 things, all 3 important)
The first time  I saw fire in a PC, was , when the silly IDE power plug 12vdc yellow wired molex pin , accidently landed on a metal steel bracket inside the PC and the PSU shoots fire inside and out ,sparks,&  smoke out the rear fan port, (yah , junk PSU)
What if  60amp rail had a 50amp short? what then. Gee 50 amps is ok on my PSU, (new) but not mister wires.
That is why ATX v2.2 limited current here to 20amps to prevent the fire, fire danger is real it is not some magic event , shorts happen and only that IC4 below can stop the fire. (The 35amps V1 shorted at 25amps is not pretty to watch)
Photo-2 (this is near modern 2006 year), Virtual RAIL PSU)
 See that reverse "Y" split below, and how the two 12v virtual rails are created (this is reality) This is exactly how this is done. (I did the data sheet math here for Amps) R61 creates an ammeter shut (classic EE)

One more  PSU below, for learning how a simple RELIC PSU work from about year 2000.
Note the total lack of OCP output current protections. (only OVP over voltage protections on 3.3, 5. 12 ) but no UVP 12v, under voltage 12v is missing here.
Photo 2: (relic) 230watt Note the dummy loads on each output , telling you there is no minimum current spec. ! Note the optical couplers IC2/3/4 keeping the high voltage from ever landing on the right output side (3kv protected)
The only fire safety are the 5A fuse on the left and UVP under voltage protection sees volts drop at any short and turns off and the 230watts limits current natually. (weak PSU's are more safe)

The best place to see what supervisor chips are used  or the makers name and p/n chip is here.(a look inside is PRIME INFO !!)

The Corsairs (a top brand) use the very good Weltrend chips for OCP, and  the expensive AX1200i use all DSP (pure software in the Digital Signal Processor does all this magic and x10 more)
The DSP is so fast is can turn off  PSU in 16mS or less,  .016 seconds. No chance of fire. (or blown up $500 CPU /GPU)
Corsair has the most models sold (and great DSP) on the shelf.
Why DSP is best ?(answers)

Deals for 2019 on DPS based PSU;   $53 on amazon. (I have on)

Thermaltake Smart DPS G 500W Digital 80+ Bronze Semi-Modular ATX 12V 2.31/EPS 12V 2.92 Power Supply 7 YR Warranty PS-SPG-0500DPCBUS-B
The monitor software that uses USB port, to PSU work great in my PC.
DSP is now and the future !

Most PSU are sold to gamers so have horrible data sheets.
The focus on HYPE and no facts about the product in details. (like OCP trip points, yes OVP but not OCP ) << my big gripe.
They avoid OCP topics and sped.  in my opinion, because they fear, Clause 6 of IEC 62368-1
One of the very first RAILS ever made  was here, and might be lethal , for radio transmitters or below the Receiving stations.  1922, (one of the first in N.America) (see the whole thing here)
They look like 4 rails , or  rail road tracks with 4 Rails.   Note the extreme redundancy here, radios went  bad often. (even after 500 hours use! or less, with the TUBES then for sure and for sure 1922 .)

Be sure  to wear your Teflon shoes and underwear.? And NEVER 2 hands at once !   This setup is very dangerous by todays standards, #1 is exposed power. wow.

version 1.  5-9-2019