Rex and the Cotton Race is the twenty sixth and final episode of the seventh season.
Rex was working hard. Summer had come, and the workload at Willie's Cotton Farm was growing quite quickly. One morning Mighty Mac were at Arlesburgh Harbour, having their vans unloaded when Rex puffed in with his load of cotton. "You look worn out!" said Mighty. "I know! The workload at Willie's Cotton Farm is very tiring around this time of year." "Oh, so you're the engine we're helping out," exclaimed Mac. "We're also helping to deliver the cotton from Willie's farm." "Really? But I don't need any help!" "We wouldn't be helping out if you could manage alone," teased Mighty. "We're much stronger and bigger." "Makes no difference! How about we race?" "You're on!"
The engines raced up and down the line from the farm to the harbour. Mighty Mac were worried; they had to wait for their line to cleared of escaped sheep. They went too fast into the harbour. Mighty had vans coupled in front of him and vans were behind Mac. The vans that were coupled to Mighty crashed through the buffers sank into the sea.
Farmer Willie wasn't happy. "You should be more careful. I need to earn a living too!" Rex felt bad. He knew he was partly to blame. "It's my fault too Sir," said Rex and explained everything. "Now listen you three. Since the Skarloey Railway runs alongside the Arlesdale Railway, I've decided to split the workload between the two railways so that Rex wouldn't have to manage alone. You three are suppose to share the work amongst yourselves." "Yes Sir." "Good. That's settled!"
Rex and Mighty Mac are now good friends, and they enjoy helping Farmer Willie deliver his cotton to Arlesbugh Harbour for shipment to the Mainland and beyond.