MTH often shows them both ways in the catalogs. Some are actually shopped photos of H.O. models. Check the stock number 20-nnnnn-1 is the hi-rail with swinging pilots, large couplers, and hi-rail wheels. As Joe stated above, the 22-nnnnn-2 are the scale-wheeled versions that have fixed pilots, don't include couplers, and have scale wheels. The MTH Premier Dash 9 (and it's AC4400W twin sibling) are nicely done.
I buy my MTH diesels with scale wheels and run them at the club. We've had some track shifting over the years due to radial swings in temperature/humidity hitting our benchwork and most of my six-axle diesels have issues with some spots on the layout. I'm a cheapskate and haven't put the hi-rail wheels on any of the locomotives, but I may bite the bullet and convert some of the six-axle units for use at the club. The four-axle diesels work fine. Of course, we're planning to change out some of the older track down the road.