We are touring Seattle in early August (4 of us) and would like to take a quick trip to see Lake Louise and Banff area. What would you recommend for getting there and back that isnt expensive? Is there an inexpensive train?

I'm no expert on this, but a little past research on this idea made me think that such a trip is neither "quick" nor "inexpensive".

I would suggest you also post on the Canada board. Lots of help with that area.

So it's over 600 miles, so more than 11 hours of driving. Obviously, the easiest would be a plane flight to Calgary and then a rental car. But this is also the height of the high season, so keep in mind that prices will be steep.

There's a train from Vancouver to Calgary, stopping in Banff, but I don't think it's particularly cheap.

This is not a simple, easy, nor cheap side-trip. Vancouver (or perhaps the lower half of Vancouver island) is a simple, easy side-trip. But if it's where you want to go, do some research and see what you can find that's affordable to you.

We have made the drive from Seattle to Banff several times. If is a very long day, but sooo pretty! With 4 people, I would guess that a rental car would be the cheapest.

The train from Vancouver which stops in Banff/Lake Louise is the privately-run Rocky Mountaineer and that will not be "quick" in any way, shape, or form

The Rocky Mountaineer is neither quick or cheap.
The cheapest option involving a train is to take the Empire Builder to Whitefish (arriving in the morning). Rent a car that can be taken into Canada. Everybody have passports?
Drive over the Going to the Sun road to St. Mary.
Head north through Waterton and Calgary.
The ViaRail Canadian runs 3 days a week and stops in Jasper.
All of the northern Rockies have great scenery no matter which side of the border you're on.