Insufficient data to solve it so far. The US-Canada border is straight for hundreds of miles only along the 49th parallel from Manitoba to British Columbia. There's a stretch of about 150 miles between Quebec and Vermont that's straight, but the first post in that link specified "hundreds of miles," which disqualifies that location. Or perhaps the poster was a little inaccurate? It does look more like the terrain between Quebec and Vermont than it does anything in western Canada. It's clearly not Manitoba or Saskatchewan, there's no terrain along the 49th parallel in those provinces that looks like that, so it'd have to be Alberta or British Columbia. But it doesn't look like the mountains I've seen while flying between Calgary and Vancouver either, they're much more angular and pointy.
Other factors: it'd be very unusual for a plane to precisely follow either the 49th parallel or the Quebec-Vermont border for a long time, those paths don't join any major airports. The only reason to do so would be to fly in the jetstream, so it'd have to be an east-bound flight, but the jetstream is rarely straight for hundreds of miles either. Nature doesn't usually operate in straight lines, only people do.
My first thought was that the picture's a Photoshopped fake. I saved it and had a close look at it, and there are places where the line doesn't seem to quite follow the terrain. Besides, for the line to appear that wide from 35,000 feet, it'd have to be many miles wide on the ground, and as far as I know we don't clear vegetation along the border like that, or paint the landscape to mark it. Double besides, the line is white, suggesting snow fills it, but there'd have to be a lot more snow elsewhere in the image to justify that belief. I think it's a fake.
Or a giant slug trail. What, you've never heard of the giant mountain slugs in British Columbia? Whole towns have been engulfed...