Canada is the great north low fare territory for cheaper flights to Europe. Delta will fly you from Vancouver to Amsterdam outbound on a KLM nonstop flight in June for $605 USD round trip. Delta will have you connect in the USA, Minneapolis MSP or Seattle SEA on the return for the low fare price.
Delta wants over $1,400 round trip from from Seattle. Train fare is not nearly that much for Seattle to Vancouver.
Except for WOW Airlines in LAX, there are no airlines in the western U.S. right now with June fares to Amsterdam under $1,100 round trip. WOW will cost you around $900 after bag fees.
You can buy an Air Canada ticket today to fly LAX to Vancouver for $199 round trip in June and July.
Google Flights Seattle to Amsterdam Delta/KLM $1,426 June 22-July 7, 2016
Delta Air Lines / KLM Vancouver YVR – Amsterdam AMS
$604.89 USD ($792.21 CAD) Wed June 22 – Thu July 7