Speaking of the Canadian, when I booked my trips around 2008 and 2011, VIA rail had a special deal section that listed last minute (i.e. next six weeks or so) specials on both regional trains and all classes of service on the Canadian. I've looked around and haven't found anything like it. They have the 25% off Tuesdays but I think it is only 10% off sleeper IIRC, which isn't much of a bargain. Does anyone know if they are still offering any larger discounts off season?
The Sleeper Plus sale page, which is what you are referring to, was essentially a "distressed inventory" sale for select departures, accommodations, and segments, and inventory was released to it, as you point out, about 6 to 8 weeks out.. It got you about 40% off of the undiscounted fare for the fare season it was offered in.
While any accommodation could show up on it, the ones that showed most often were berths.
I doubt it will come back this year. It might at the end of summer. That is just a guess.
VIA's overall pricing, aside from the Sleeper Plus and Discount Tuesday distressed inventory sales, is much simpler and easier to parse than Amtrak's yield management "bucket" system.
There are two fare classes non-discount and discount. Discounted fares have some some not very onerous restrictions and inventory allocated to "discount" is limited.
There are three fare seasons, peak, shoulder, and off peak. I know off peak starts on November 1 because I usually take it in the Fall. Shoulder starts about mid September. Not sure of the spring dates. March is still "off peak" though, my fare for my recent trip was clearly based on off peak.
Shoulder season fares are not that far off of peak fare. Maybe like 10% lower at most, maybe not even that much. Off peak is a LOT lower, like 25%.
An off peak discount fare is like 35% lower than a peak undiscounted fare.
The best deal is to be had by riding it once a year, the full route, in a room. It will cost more than the $1000 CAD that puts you in the "Privilege" elite tier of VIA's Preference frequent traveler program. Privilege level gets you a 50% off (undiscounted) coupon for Sleeper Plus. The only catch is there has to be "discount" inventory available for the departure you want.
I just did a round trip Vancouver-Toronto-Vancouver using that coupon for a price that I could not have gotten near on Amtrak these days for an equivalent length trip (8 nights).