Rail is a trip. Road is an itinerary. Choose based on who is in the car.
Take the train if the journey is the point, if someone in the group should not be driving mountain passes, or if you want the Fraser and Thompson canyon sections that no road follows closely. Take the car if you want Moraine Lake at sunrise, Peyto in the afternoon, and the freedom to stop at a farm stand in the Columbia Valley.
The rail options
VIA Rail's Canadian runs Toronto to Vancouver through Jasper and is a Crown corporation, which means it is Canadian owned by definition. It is a scheduled train, not a tour, so it is cheaper, slower and less predictable on timing. The Jasper to Vancouver leg alone is the highlight and can be booked as a segment.
Rocky Mountaineer is a private Canadian company running daylight only, no sleeping car service between Vancouver, Kamloops, Banff and Jasper. You sleep in hotels. It costs multiples of VIA and delivers a curated glass domed experience. Both are Canadian owned, which is unusual in the premium rail category worldwide.
The road: the Icefields Parkway
Highway 93 between Lake Louise and Jasper is 232 kilometres and takes three hours without stops, which nobody does. Budget a full day. Bow Lake, Peyto Lake, the Columbia Icefield and Sunwapta Falls are the fixed points. There is no fuel between Saskatchewan River Crossing and Jasper, so fill up. A Parks Canada pass is required for the whole corridor.
Moraine Lake is private vehicle restricted. You reach it by Parks Canada shuttle, a commercial shuttle or a bike. Book the shuttle the moment your dates are set, because the sunrise slots are the first to go and they are the reason to go.
The Kootenay alternative nobody books
If Banff and Lake Louise are full, drive Highway 93 South through Kootenay National Park to Radium, then north up the Columbia Valley to Golden. The mountains are the same range, the towns are Canadian owned rather than internationally branded, and the hot springs at Radium and Fairmont are open year round.
What to bring rather than buy there
Mountain town retail is a captive market. A Canadian made wool layer, a real thermos and a hard cooler bought before you leave will cost less and last longer than the equivalent bought in a resort village. Fill the cooler at a Farmers Market in Calgary or Kamloops on the way in.
Season notes
The Icefields Parkway is open year round but requires winter tires or chains from October 1 to April 30, and it is not routinely plowed overnight. September is the best month: the wildflowers are gone, the crowds are gone, the larches turn gold in the last two weeks, and the shoulder season rates start.