Aquarium tank wagons are closed wagons with clear sides and are filled with water. They are used for the transportation of live sea animals such as sharks, octopuses, fish, ect. They were only seen in the books Down at the Docks and Thomas and the Shark.
Thomas first saw these wagons while he was looking for a way to help with preparations for the ocean show at Tidmouth. He admired the squid in the first one, but was frightened by the shark in the second. Salty who was coupled up to the tank wagons told Thomas the two sea creatures were meant for the aquarium. Percy puffed pass Thomas and Thomas tried to tell him about the special cargo Salty has, but Percy was in a hurry and puffed off quickly. Percy puffed up to the tank wagons, and was also frightened by the shark. He was so frightened that he caused a huge accident. After Thomas helped clean up the mess, he went with Salty to look at the wagons when the shark winked at him.