The answer is likely, some of this, some of that, and a whole bunch of the other….
You may want to read the Transport Canada
Rules Regarding Track Safety.
Class 2 is 30 mph, Class 3 is 60 mph. One major differentiator is the number of “good” ties per 39 foot segment. Another is the condition of ties at rail joints. Other things that have different tolerances such as gauge and track surface geometry is assessed by automated inspections - those big track test train thingies. Defects in the rails, and overall rail wear, likewise.
Grade crossings are a key spot where track geometry goes wonky - different roadbed conditions plus the pounding from auto traffic may cause low spots.
And there are differences in the frequency and type of inspections - meaning CN has to agree to inspect more frequently and more intensively before the higher speed can be allowed.
I have no specific inside information on what is being upgraded, but it’s pretty clear that over the GEXR era, many things were downgraded or allowed to get out of spec and now they have to be brought back up to snuff.
- Paul