What is this?

I happened to come across this photo of a very unique engine:

https://railpictures.net/photo/755719/

Does anyone know anything about this loco? What is it?

NS has two active track geometry train sets.

The one shown here consists of NS Research platform 38 “The Brick”, which was rebuilt using the frame and trucks of former NS/NW EMD SD40 1620. It contains sensing and measuring equipment for track geometry and rail profile (rail wear), as well as the the power supply for itself and NS Research car 36, and is weighted to deflect the track. The Research 36 car contains the recording and processing equipment for the data supplied by NS 38.

NS 38/36:
http://www.rrpicturearchives.net/showPicture.aspx?id=5448474

The second set includes NS Research 34 platform, which was built on the frame and trucks of what was originally N&W SD35 1530. It is mated with NS Research car 33. This pair works in the same manner as described

See linked article from site TRAINORDERS.Com

@ https://www.trainorders.com/discussion/read.php?2,5351847

What you have is apparently, one of two sets of NSRR Track Geometery Measuring trains. Information on both pairs is on the linked article; Above.

So, it’s not self-propelled? It needs to be pulled or pushed by a loco?

That’s correct. It’s not a locomotive, it was just rebuilt using the frame of one.

Side view; note the large ‘cab’

https://www.railpictures.net/photo/673275/

https://www.railpictures.net/photo/673275/

How many people typically ride in one of those?

My GUESS would be two to four. That being said, I have no idea what their job responsibilities would actually be.