As we know, US citizens can cross into the USA without a passport if they can prove citizenship. I am just curious, what if you don't drive? Starting from Toronto, I can see trains both to Windsor and Niagara Falls. From Windsor, you can take the Tunnel Bus across (and curiously enough, it emphasizes only proof of citizenship is necessary). From Niagara Falls, you can walk across the Rainbow Bridge. Once across, you can get to DTW or from Niagara Falls, BUF or IAG.
From Vancouver, based on my own experience, Amtrak, Greyhound, BoltBus, QuickShuttle all want to see a passport before letting you on (edit: Quickshuttle let me board last month both directions without asking for a passport!). You can't get really close to either of the US border crossings with Translink (it's a mile from 8th Ave) and if you do walk over, there isn't a lot of public transit from the other side to BLI or SEA either.
So, what solutions exist if you are in a situation like this and you don't drive and want to get from a Canadian airport to a USA airport?