I've also never seen a double decker trolley car. Yes, people do use the top in decent weather - although there's only about 3-4 months during which it's nice enough to use.
The Detroit People Mover is an elevated light rail system which runs through Downtown Detroit. It was originally intended to be the hub of an extensive light rail service, but politics & recession in the 70s got in the way, and so only the 3 mile downtown loop was ever built. The original plan was for it to be part of a multi-billion dollar regional transit system consisting of 2 regional commuter rail lines, 4 subway lines, & an improved downtown bus service. Currently a couple of the stations are under construction, so it does not run in a complete loop, but rather goes back and forth from end to end. Construction should be complete soon, & the People Mover will again run the full loop. If you're ever in Detroit, ride around the whole system 3 or 4 times - it's one of the best 50 cents you'll spend. You'll get to see Detroit - both the nice parts & the, well, not so nice parts that you might not want to explore on foot. The entire loop takes about 14 minutes, & the trains run 2-3 minutes apart during peak times & during major events (such as the Detroit International Auto Show).
In the first year of operation, peak daily ridership was 54,000; today it's only about 5000. Hopefully ridership will increase with the revitalization that's currently taking place.
Here's a map of the People Mover System. The map isn't all that good, but it gives you the idea.

The Greektown Station

The train runs by the hole where Hudson's used to stand...it's now yet another surface parking lot for Downtown Detroit!

Grand Circus Park Station

Approaching the Times Square Station

Platform at the Financial Center Station

Artwork at the Financial Center Station. Each station has unique artwork by a different artist.

Train Crossing Jefferson Ave.

Several of the stations are Integrated into buildings, Including the Millender Center.

Milldender Center Platform.