Ferroequinology's video: Chasing the Hi-Rail Van

Video from August 2017 A strange hi-rail van came by, and we decided to follow it. For those who don't know, the hi-rail truck following it would be considered 'normal'. The only hi-rail trucks I've ever seen are that type, or a larger version. But never a van. This strange van also seemed to be dragging a small cart with some kind of measuring equipment consisting of lots of tiny wheels. They had to slow down to a crawl over the diamonds and switches while all the tiny wheels went over the flange gaps. As it crossed the 3x2 diamond in DesPlains, heading towards Chicago, a few clanging sounds can be heard. The workers in the following truck stopped to tighten a few bolts on the diamond. I didn't realize this, but less than 2 weeks later, Union Pacific worked for over 48 hours straight to completely replace that diamond. A time-lapse of this can be found on their official Youtube channel. I'm not sure of the exact purpose of this truck, but it appears to be measuring the track. Maybe making sure it has the correct gauge, or maybe testing how bumpy it is. If that's the case, why not just send the geometry train? I could only find one other video of this truck on YouTube, and it was shot from very far away.