If we are taking the train as part of our land tour (Denalit to Mckinley to Whittier) would we want to go on a train again for the White Pass Scenic railway?
Yes, most definitely. The White Pass & Yukon Route (WP&YR) -- a relic of the Yukon Gold Rush -- is a very different railroad in a very different locale set in a very different era passing through very different terrain than the Alaska Railroad. If you have not been to Skagway before, the ride to Summit Lake on the WP&YR is not to be missed, even if your land tour includes the Alaska Railroad!
BTW, I also recommend catching a performance of "The Days of '98 Revue Starring Soapy Smith" at the Eagles Hall in Skagway. Most of the cruise lines have at leat one tour that includes this play, and you will be able to do this and the ride on the WP&YR -- one in the morning and the other in the afternoon.
These are completely different rail experiences. The Alaska RR is transportation based, on your portion, it just about parallels the Parks Highway most of the trip.
The White Pass scenery is grossly superior, but the history and engineering are definate highpoints to also consider. It's not for everyone, determine if of interest for you?? A great option are the combo tours with a bus into the Yukon via the Klondike Highway and train one way.