The longest distance passenger train service in Canada is Via Rail's Toronto ⟺ Vancouver train [The Canadian] (Wikipedia entry here). The trip takes four days and four nights, and is about 4,400 KM. The notes below are based on a trip I took from Toronto ⟹ Vancouver on September 1 (2019) during my 6-week sabbatical from Adobe. This write-up is intended to help others who may be planning a similar trip.
With Toronto ⟹ Vancouver air travel now taking about four hours at about CAD $500-800 (two-way) (compared to 4 days/nights for $1,700 for a one-way subsidized sleeper plus ticket), chances are that you may only do this once in your lifetime, and you want as few screw-ups as possible.
Via Rail is what we call a "Crown Corporation" in Canada. Its exclusive focus is passenger rail service, and is subsidized by the federal Government of Canada.
Before you go on this trip, it's worth checking out (as part of your research) some really useful content available out there both online and in the form of books you can purchase.
Other than driving, this is the second best way to take in the geographic vastness of Canada. This includes the northern Ontario boreal forest (Toronto-Winnipeg), the Saskatchewan/Alberta prairies (Winnipeg-Edmonton), and the Canadian Rocky mountains in Alberta/British Columbia (Edmonton-Kamloops). The highest elevation (according to myTracks GPS readings) the train reached was 1,145 meters above mean sea level, and the highest speed we hit was about 130 KM/hr which happended mostly in the prairies. The train speed was mostly 60-80 KM/hr in the northern Ontario boreal forest and in the Canadian Rockies.
Because the tracks are owned by Canada's freight railroads (mainly Canadian National - CN), Via Rail trains have to yield whenever the freight trains need to pass. Because of this, [The Canadian] is reported to be routinely late although in my case the train arrived in Vancouver three hours earlier than scheduled (5 am, instead of 8 am).
Each [Sleeper Plus] car has two shared bathrooms and a shower with an adjoining change room. These shared bathrooms are used only by the [Sleeper Plus] class passengers of which there will only ever be six at the most (six sleeper berths, three pairs). The others in the car will have either 1-person, or 2-person cabins which all have their own private toilets/wash sinks.
They provide towels, soap and shampoo. Hot water is comfortably hot.