New Delhi, April 14 -- On a recent sweltering afternoon in the Cuban capital, dozens of commuters on bicycles, scooters and electric motorcycles gathered in a tidy row at the entrance of the Havana Bay Tunnel. They were waiting for the Ciclobus, a bus specially fitted to take people - and their rides - through the underwater tunnel linking Old Havana to the eastern side of the island.

The diesel-powered bus can accommodate around 60 travelers and their vehicles, making enough trips to transport more than 2,000 people per day. It features a front seating section, but half its metallic frame is an open bay for cargo. Riders enter via a specialized ramp and stay with their vehicles for the duration of the trip, holding onto wall-mounted gra...