Rock 'n' Roll is the tenth episode ever made in Season 4.
Rusty is concerned that Duncan will derail at a dodgy section of line near Cros-ny-Cuirn due to his "rock 'n' roll", but Duncan refuses to take advice from a "smelly Diesel", and matters are only made worse when James backs Duncan up and tells him of the time he supposedly sent Diesel packing. Duncan is full of admiration, and starts to "rock 'n' roll", causing him to derail. Rusty is furious and refuses to help, until Skarloey reminds him of the passengers. Rusty then helps Duncan get back on the line and, that night, the two engines become good friends.
- When Skarloey meets Rusty, he is on (viewer perspective) the left side of Rusty. Further down the line, they seem to have switched tracks.