The win follows a pitch against undisclosed agencies.
Smarter Communications, which won the creative account last September, also previously handled media.
Rocket will work on a range of campaigns, starting with the launch of South West Trains' "advanced ticket" product, which offers tickets at cheaper prices the earlier they are booked.
Other campaigns will include activity to promote rail travel between South-West counties and London with a particular focus on travel to and from the growing airport at Southampton and on "two for one" ticket promotions.
Lucy Thornton, the head of marketing at South West Trains, said: "Rocket has demonstrated a very strategic and innovative approach to communicating with audiences, and a practical approach to return on investment."