Hamburg's lost over-and-under monorail

Hamburg's Cabintaxi was an "over and under" monorail design that ran personal monorail cars in both directions, with counterclockwise traffic on one level and clockwise on the other. It looks like it never got deployed, but it's one sweet retrofuture design for urban transport:
The computer now takes over completely. It regulates our speed, senses the position of any car ahead to maintain a safe headway, and holds us at intermediate stations only if the track is temporarily occupied. We’re programmed for travel to the selected destination by the most direct path. Upon arrival, the car is released for immediate use by other travelers.

“Our over-and-under guideway is a big space-saver and cost-cutter,” MBB’s Gert von Lieres told me. “A two-level guideway can fit into narrow streets that couldn’t accommodate parallel rails, and there’s less clutter in the streets from support columns. Construction is simplified and thus relatively cheap.”

It’s also a quiet system; the cars glide at about 22 mph on rubber-tired wheels. And it shouldn’t be affected by weather; the base, suspension, and guidance tracks are fully enclosed within the beam. “This protects them from snow and ice,” von Lieres said, “while the lateral rollers eliminate any risk of derailment. They allow tight corners—a turn radius as small as 100 feet—meaning greater versatility in urban routing.”

Over-and-under monorail — a single beam tor two-way taxis (Jul, 1980)