The Jargon File (version 4.4.7, 29 Dec 2003):

big iron
 n.

    [common] Large, expensive, ultra-fast computers. Used generally of 
    number-crunching supercomputers, but can include more conventional big
    commercial IBMish mainframes. Term of approval; compare heavy metal,
    oppose dinosaur.


The Free On-line Dictionary of Computing (30 December 2018):

big iron
heavy metal

    (Or "heavy metal [Cambridge]) Large, expensive,
   ultra-fast computers.  Used generally of number crunching
   supercomputers such as Crays, but can include more
   conventional big commercial IBMish mainframes.  The term
   implies approval, in contrast to "dinosaur".

   [Jargon File]

   (2000-11-09)