Thomas, Percy and the Post Train, retitled Thomas, Percy and the Mail Train in American releases, is the twelfth episode of the third series. It is based on the magazine stories The Post Train and After the Last Train.
Late at night on the Island of Sodor, after all the other engines have gone to sleep in their sheds, an engine can still be heard in the distance carrying the Post Train. Thomas pulls one train while Percy pulls the other as the loads are too much for one engine to pull alone. Letters and parcels are loaded into the trucks at both harbours before Thomas and Percy begin their journeys, travelling through the now silent stations making deliveries. Sometimes the light of a full moon on a clear night will light the way for them, or sometimes it will be so foggy that Thomas and Percy cannot even see the stars but lamps will always be out to guide them on their way and regardless of the weather, the two tank engines are happy to be entrusted with such an important job.
One night there is trouble, as Percy is left waiting due to the ship from the mainland being delayed. However, Henry finally gets the letters to him and the Post Train is filled. Percy tries his best to make up for the lost time, but he is still very late and finishes his job after the sunrise. Percy does not mind this too much, appreciating the chance to see the island early in the morning with no-one around... but he is not alone long as Harold lands and teases Percy over how late he is with the post. Percy is too tired to explain to Harold what happened and he finally arrives back at the sheds just as Duck is getting ready to head out for the day, falling asleep as soon as he enters his shed.
Percy and his driver decide to set off earlier than usual that evening. However at the station they find Thomas who has some bad news: the man in charge of the post has made a complaint to the Fat Controller about Percy being so late. Although the Fat Controller understands that it was not Percy's fault, the man would not listen. The two decide to be extra quick that night when Harold lands telling the two engines that the Post Train may be scrapped and the deliveries will be made by him instead. He boasts to the engines that air is quicker than rails before taking off. Both Thomas and Percy are indignant and proceed on their way.
That night, everything runs perfectly and both engines make very good time on all their deliveries. At a station, Thomas and his crew notice a man who had missed his train waiting cold and alone. Thomas' driver offers the man a ride if he does not mind being uncomfortable. The man is more than happy to accept.
The next day as Percy pulls his afternoon passenger train, he spots Harold looking miserable in his airfield. Harold explains he has been grounded due to unpredictable wind. Percy takes the chance to get his own back for the earlier teasing by telling the helicopter that rails work wonders. That evening, the Fat Controller meets Thomas and Percy and shows them a letter from the man who missed his train. He says that the man was very thankful to the two for their help and the Post Train is the pride of the line. This makes both engines very happy indeed.