Why would you compare discount class, off-season to middle bucket? Wouldn't it be low-bucket to be a fair comparison?
I am comparing what would be most commonly available and relatively easy to get. VIA pretty much does not do yield management except in very limited ways.
VIA has 3 travel seasons, peak, shoulder, and off-peak with different fares for each, off-peak is about 30% or so lower than peak.
There are 2 fare classes, discount, which has some change restrictions, and regular (or non-discount) which does not. Discount class fares are about in the neighborhood of 15% off regular. Note that "discount" is due to more restrictive fare rules, not primarily yield management algorithms. Fare rules are the same for all Amtrak buckets.
The only real yield management VIA does is the amount of inventory available in the "discount" class is limited.
I am comparing off season to middle, because a middle bucket would be relatively easily available in the off-season. Discount class is likewise readily available if purchased in advance.
Had I been comparing peak season rates, I would have used Amtrak's high buckets.
The only thing I would use low bucket as a comparison would be against VIA's distressed inventory "Sleeper Plus Sale" page offers. Which are only offered on specific dates, endpoints, and accommodations. The general scarcity of low buckets and (relatively) low prices make those a rough equivalent in my book.
VIA and Amtrak fare structures are radically different and there really are not apples to apples comparisons available. I used more common availability of buckets in a season rather than labels, since VIA's fares vary by season, not day to day as Amtrak's can. You can get a discount class fare easily by booking in advance. Low buckets can be elusive even in the off season. Middle bucket is generally more available.
In short, my rough equivalency is:
VIA Peak Season = Amtrak high bucket
VIA Off-Peak = Amtrak middle bucket
VIA Sleeper Plus Sale = Amtrak low bucket
I think it is a reasonable methodology for radically different fare structures where there are really no direct equivalents. You are free to disagree.