"The Picnic" was a passenger train on the Mid Sodor Railway, which carried tourists visiting the line to their designated picnic areas. It was Duke's special train, and he always pulled it even if he felt poorly.
The "Picnic" was commonly used to take summer visitors around the railway to wherever they wished to stop to have their picnic. After they finished, they would wait until Duke returned for them, and would be taken back to Arlesburgh so they could fetch their boat. It was Duke's favourite train on the line, and he always pulled it, even if he was short of steam.
Duke once double-headed the "Picnic" with Falcon when he was learning about the Mountain Road. However, as they were leaving one of the tunnels along the stretch of track, Falcon derailed and hung dangerously near the edge of the cliff. Thankfully, Duke managed to hold onto him long enough for the breakdown gang to arrive, and they got the train to the top station safely.
On one occasion, Duke felt short of steam as he was pulling the "Picnic". Although he tried to keep going, he was unable to do so and had to come to a stop. In the end, Falcon was the one who took the train back to Arlesburgh, while Stuart took Duke back to the sheds with Falcon's train. (RWS; Duke the Lost Engine)
Shortly after the Peel Godred Branch Line opened, the Mid Sodor Railway began facing hard times. In an attempt to keep the company going, the bogey saloon coaches used for the "Picnic" and Boat Trains were sold, presumably marking the end of the "Picnic" run altogether. (Book; The Island of Sodor: Its People, History and Railways)