My guess? A lot of that freight is normally moving intermodal anyway, shifting to OTR when there is equipment issues.
My guess is you're talking about mostly out of the Dallas area, and that rate isn't sustainable, really. My guess is that even though shippers don't want to pay even that ... that really the right rates are between $5500-$6000 depending on origin to Calgary, maybe a bit more to Edmonton. If the freight pays that much ... it's probably because there is a gong show on the rail.
I've heard there is a major glut of trucks in Texas going in pretty much any direction.