P.S I've cut guard rail with a Quikie saw no problemPlasma cutter would be the meow. I welded a lot of sawdust pipe and was told to drink milk to ward off metal fume fever.( galvanized, zinc oxide) I don't remember it ever bothering me but I used to drink a lot of milk anyway.
A guardrail to be used for a pasture fence for cows? Barb wire or an electric fence is easier, quicker and works pretty well well over a 100 years.
Speaking from experience forget the sawzall and cutoff wheel. The rail I have is 3mm thick and 18 1/2" wide.. with the wave pattern it measures 12 3/16" wide and is 3 1/2" overall "thickness" while I have no clue as to the alloy it is NOT mild steel. I ended up taking a torch to it. A plasma may work but that wave pattern may limit the torch head from getting in there unless you flip the piece.
Good luck man.. took me half a day to figure it out and get the piece i needed cut to size.
I don't know what blade or saw the other posters used to cut thru a rail in 20 seconds but when i tried dewalt abrasives in my 3hp skilsaw.. ate the blade up AND since you only get a tad over 2 1/2" depth.. that blade gets smaller the more you cut and you still need to flip that 10ft beast over and do it again...
Big difference using a demo saw like this...
https://www.stihlusa.com/products/cut-off-machines/professional-cut-off-machines/ts420/
Well ya got me there but show of hands.. who has one of those?? At what 6-700 bucks i aint buying one... might be able to rent one for 75-100/day.. and for the record.. i aint putting an abrasive wheel on that then standing over it while cutting corregated guard rail.. but thats just me. The reality of the situation is this a cutting torch is the most reasonable way to cut it.. or just splurge and get a waterjet table