Part 2 of my guide to Dukes Prefer Blondes (originally posted at Two Nerdy History Girls).
My characters in Dukes Prefer Blondes spend time in hackney coaches and hackney cabs. You will often find the terms used interchangeably, as though they were the same thing. However, a hackney cab was quite a different article from a hackney coach. The cab was a two-wheeled, one-horse vehicle. It held only two passengers, and seemed to be generally regarded as a mode of transportation for those who liked to live dangerously. It was called a "coffin cab" not merely on account of its coffin-like shape.
Leigh’s New Picture of London for 1834 briefly explains the difference here. You can read about them here in Omnibuses and Cabs, Their Origin and History, which includes excerpts from Dickens’s lively descriptions.