visiting Seattle WA Sep 08, can you travel by train to Vancouver BC, whats the travel time, adult return cost?

Yes, you can travel by train from Seattle to Vancouver BC.

Amtrak's Cascade line.
Current pricing is $28 each way.
Approximate travel time is 4-4.5 hours depending on the timing.

But note when looking into this that most of the Amtrak offerings are by bus, not train. There's only one scheduled train per day each direction (ETD Seattle 7:40 AM, ETD Vancouver 6 PM). And even if you buy tickets for the train, there's always the possibility of a washout on the tracks that will put you back on the bus anyway.

And passports are strongly recommended for the border crossing!

i don't know what kind of budget or how much time you will be able to spend, but a really cool alternative would be to ride the victoria clipper up to victoria, bc. from there you can then take a sea-cab to vancouver, bc. by going this route, it would give you a better overall experience and is always cool to take the water route. while it is a little more expensive, you will have plenty of time to plan your trip and arrange accommodations as needed. below is the clipper link. hope this gives you another option to consider.