We had a wonderful holiday in Canada which we arranged through Just America (www.justamerica.co.uk). We flew from Manchester to Toronto, had 3 days there including fantastic day trip to Niagara. We then flew Toronto to Calgary and picked up a hire car to drive to Banff and then on to Jasper, Kamloops and Vancouver. We flew home from Vancouver.
You'll see that we didn't actually do the Rocky Mountaineer. (I wanted to but Mrs ZZ over-ruled me!) As it turned out it wasn't such a bad move as the weather around the time that we would have been on the train was dull and misty. Speaking to folk who had done the trip from Vancouver to Kamloops, they said much the same as Pleuni. (Scenery - what they could see of it - was very "samey" and they felt the whole thing was over-hyped.)
As I said initially, we had a wonderful holiday. Canada is fantastic, the people are tremendously pro-British and love our royal family. We met some Native Canadians (or whatever the PC term is now) who told us they loved Coronation Street and treated us like celebrities when we told them that we lived only 40 miles from where it was made!
JNO makes a good point about the boring bit across the Prairie Provinces. I suggest that you have a look at air and train timetables to see how long those journeys take. Better still have a good look at Just America's website and then give them a ring. Mike, the boss, has been there. done that. He'll tell you that a very large percentage of their holidays are tailor made to customers' requirements. we have had three holidays with them and our dughter and husband have had one (very complicated but absolutely hitch free!).
Enjoy Canada! I envy you!