Sleeper replacement consist

I would love to be able to show pictures of this but couldn't get any -   The local CN district has been doing a major tie replacement (about time and topic for another day...) along a section of heavy freight service (Burlington, points north).  The consist was quite interesting, and about the only thing I recognized was the caboose (!) and the loco.  Between the two was a long (several cars long) high metal framework that looked like nothing I had seen before - I couldn't see the cars at rail level because of slope and bushes.  It looked like it might be used to carry track but seemed way too light, and it didn't appear to carry anything - just a very long straight open frame.   My guess is that it might be used to measure level along a section of track in order to get ties at the right height.  What would it be?