designed by Lobsang and The Black Corporation, and piloted by Lobsang, Joshua Valienté with the help of Shi-Mi, the Mark Twain was a dirigible airship designed to sail through the infinity of long Earths, recording and measuring what could be found there.
It very nearly foundered in the unexpected Gap but lived to tell the tale, despite suffering severe damage. all subsequent models, called twains in honour of the first in line, are now routinely built to be able to withstand at least temporary exposure to deep space and can step through worlds far faster. The USS Benjamin Franklin, captained by Maggie Kauffman, is a typical example of the most technologically advanced twain.
Other Twains mentioned in the books have names like the USS Akron and the USS Macon. These were part of the US Navy's airship fleet and seen as the apogee of military airship technology - the Akron could even launch and recover conventional biplane fighters in the air, making it an airborne aircraft carrier - but were discontinued along with airships of all military powers after the Hindenburg disaster in the late 1930's. In 2014, the US Navy and USAF are seriously reconsidering the reintroduction of airships (with safer helium gas) to replace an aging fleet of AWACS planes. The US Coastguard is also interested in the idea as a means of efficiently monitoring the USA's sea borders.