Glad to hear my thread actually had readers. The train looks cool but it will be long, and looks like 2 days. Vancouver to Kamloops, at least by car, was pretty forgettable. But Kamloops to Banff should be pretty cool.
There is also a half day train from Banff to Jasper. I don’t know about the train, but the drive was spectacular. I would recommend investigating Banff to Jasper, overnight in Jasper, and back to Banff with plenty of stops along the way. The glaciers, icefields, lakes, river, tiaga area, are all beautiful.
If I did my trip all over again, it would be to drive to Jasper, camp there a night or two, then to Banff, and eventually back to Vancouver/seattle. We drove to or through Lake Louise multiple times.
The kicking horse river white water rafting, if it’s open that early, will be a helluva ride. It was pretty adrenaline inducing in August, so the spring run off might be too much for some folks. My kids loved it and was the highlight. Well, except for Bullshit Mountain, which I highly recommend you skip.