During the late 19th and early 20th centuries, mechanical power sources revolutionized public transportation. It started with the trains, then moved to streetcars, underground trains, and busses. Thanks to steam, combustion engine, and electric power, public transportation could offer a high transportation volume, allowing the cities to grow even more rapidly. Today, more than 100 years later, we still use the same type of transportation means. Today, we face different challe