Shoot, they look like dually offset wheels. http://www.flickr.com/photos/photohound/310708874/sizes/l/
I think they're running DRW axles (wider) without the outer tire on the back with SRW beds. Dang. http://www.flickr.com/photos/setandcentered/2311091267
Those trucks drive by the shop I work at all the time since we're right next to a switching yard and I think you're right.
Actually, the rail tracks are 4 ft. 8 1/2 inches apart, the truck axles are standard SRW, and the wheels are offset inward to get the 56.5 inch track. On a standard dually truck, the inside duals ride on the 56.5 in. apart rails.
yup you are right roadranger. they have normal SRW axels just different offset rims. our 450 at work sets the inside duals right on the rails perfectly and our single wheel trucks just have those different rims on them. And a commercial truck like an international boom truck such as the one i drive for UP the inside duals sit right on the rail too.