Double Decker Trams have been around for years. I know at least one in Blackpool, England.
These, however, are more modern and have been proposed since 2012 for Stockholm to be built 11 years from now.
The best thing about them is that they can carry as many people as they would if two cars were connected, but would take up a smaller footprint.
Just thought I'd see what you guys thought about it.