Simon Cowell -- 'X Factor' Trailer Transforms Into Kick-Ass Emergency Room

Simon Cowell's enormous "X Factor" trailer isn't just pimped out -- it's a freakin' MASH unit ... fitted with better medical emergency equipment than your average doctor's office!

A source close to Simon tells TMZ the trailer now has something called a "Ready Cabinet" ... which transforms from an ordinary piece of furniture into a virtual hospital -- with a camera to examine wounds, a scope to check the inner ear, and a vitals monitor.

The rig is even ready for bad tickers -- it has an EKG glove that looks a lot like an oven mitt, but costs an incredible $4,000.

We're told Simon's in good health, and doesn't need this stuff -- but the company that makes the cabinet needed an over-sized testing ground -- and Cowell's huge trailer fit the bill. The prototype will eventually be used for disaster relief.