'On The Road Again' was the theme song to the movie 'Honeysuckle Rose' and it came about during a flight to Hollywood when the producer asked Willie Nelson if he would write a song for the movie.

About 45 minutes later, Nelson had written the entire song on an airline's barf bag. The song 'On The Road again' is pretty much a song about a musician who can't wait to get back on the road  again touring and playing music

The song features a 'train beat' to it, giving it the feeling that you're on a train and constantly on the go. Check it out above and I'll bet you can sing every word of it.
Source: SongFacts.com