The Lost Road is the name of a time-travel work that Tolkien only wrote four chapters (and assorted notes) and is entirely integrated with 'the main mythology'.
It tells of the voyage of Ælfwine and his trip to Tol Eressëa along with other characters journeys in later times. Much of this story was later influenced the details of the similar The Notion Club Papers (with at least one of the characters referencing the general events of the The Lost Road).
- I began an abortive book of time-travel of which the end was to be the presence of my hero in the drowning of Atlantis. This was to be called Númenor, the Land in the West. The thread was to be the occurrence time and again in human families (like Durin among the Dwarves) of a father and son called by names that could be interpreted as Bliss-friend and Elf-friend. ..... It started with a father-son affinity between Edwin and Elwin of the present, and was supposed to go back into legendary time by way of an Eädwine and Ælfwine of circa A.D. 918, and Audoin and Alboin of Lombardic legend, and so to the traditions of the North Sea concerning the coming of corn and culture heroes, ancestors of kingly lines, in boats (and their departure in funeral ships). ..... In my tale we were to come at last to Amandil and Elendil leaders of the loyal party in Númenor, when it fell under the domination of Sauron.
The importance of the story is that it was one of the earliest introductions to the story of the fall of Numenor.
The book's title is a reference to the Straight Road (which is also known as the Lost Road or the Straight Path)