Burlington sits on Interstate 5, about 80 km north of Seattle and 40 km south of the Canadian border at Blaine. It is a straightforward drive from Seattle-Tacoma International Airport (about an hour in light traffic, often longer).
Amtrak's Cascades and Coast Starlight trains stop at the Burlington station, connecting to Seattle, Bellingham, and Vancouver. Greyhound and Flixbus also serve the I-5 corridor. If you're heading for the North Cascades, Burlington is effectively the last town of any size before Highway 20 begins climbing into the mountains.