The inventory of regular urban transportation modes, as explored in the previous chapters, is a rich one, but there is more yet. Over a time span of almost two centuries, there have been great necessities to find better means of mobility, and there has been urgency to do so quickly so that progress in development is not impeded. In many places growing districts have encountered physical obstacles that have had to be surmounted to capture new territories for continued expansion. Engineers and inventors of our technical age have been able to respond with any number of ideas, but, alas, not too many of the new devices have been able to survive the harsh tests of reality, prove themselves useful in actual applications, and persevere under changing conditions. Those that have been able to do all that are the principal urban transportation choices today. They are the workhorses.

But there is also a second group: modes that were dominant once but have been supplanted by better options, modes that respond to very specific needs without claiming much general applicability, and modes that appear to have great potential for service but have inherent limitations that are not always immediately obvious. These are the donkeys and the racehorses interesting and sometimes useful beasts, but not all-purpose workers.

The purpose of this chapter, with a brief survey of each special mode, is to be comprehensive, i.e., to provide some information on every urban transportation possibility in the expectation that new ideas about feasible choices...