The term twains defines the airships used to travel across the Long Earth.
The Mark Twain
In the years after 2030, when Joshua Valienté came back from what is now known as the Journey, Douglas Black (majority owner of the Black Corporation and architect of the Mark Twain) gave the twain technology to the world as a gift.
After the Journey
The twains helped mankind spread across the Long Earth, making obsolete the treks like the one the Green family did to reach Earth West 101,754.
A commercial route named the Long Mississippi, linking Datum Earth and Valhalla on Earth West 1,400,013, was created to import goods back to the Datum (like rare minerals, exotic plants, timber) and export goods to the stepwise worlds.
Nowadays, the commercial twains like the Gold Dust can travel at a speed of one step per second while twains with more advanced technology, like the USS Neil A. Armstrong II, can cross fifteen million worlds in a week.