Hi Teresa
Welcome to the Community - you've picked a terrific tour and I'm sure you'll have a super time. If you type 'Rocky Mountaineer' into the search function at the top right of the page, you'll find reviews, hints and tips from fellow JYers who have done the trip previously.
I did an earlier version of the itinerary back in September 2011 and (until Alaska overtook it recently) it was my favourite tour so far. I would say that packing layers is the key. While in the mountains it was surprisingly chilly, especially in the mornings, and it even snowed on 3rd Sept! As you will be going along the ice field parkway, I would recommend taking a fleece, hat and gloves. However, by the time you reach Vancouver you will probably be in t-shirts because from Kamloops onwards it will warm up quite a bit. The train itself is very comfortable and you won't a coat with you for those two days of the journey.
Happy to help if you have any other questions
All the best
Mandy