The tank train is a hybrid Fire Nation war vehicle, combining a land train and an armored tank. Primarily used as a means of transportation, it is designed to travel at high speeds over most terrain without use of tracks, and is capable of hauling many people and large loads of cargo across great distances.
Azula, Ty Lee, and Mai used this fast-moving vehicle to pursue Aang and his friends all over the Earth Kingdom. Every time the team lay down to rest, Toph sensed the machine's approach, stating that it felt "like an avalanche, but also not like an avalanche". The team fled from the unknown approaching danger all night, physically and mentally draining themselves from lack of sleep.
With Appa utterly exhausted, they agreed to face the machine and found out that it was Azula, Ty Lee, and Mai who were tracking them down. The three girls emerged from the tank on mongoose dragons and charged the foursome. After seeing that the three girls avoided their defensive attacks with ease, Team Avatar decided to flee.
When Toph determined that it was Appa's shedding fur that left a track for the three girls to follow them, the team gave the sky bison a bath. Aang created a false trail using Appa's fur to lead the trio off course while Katara and Sokka flew away on Appa. However, the still tired Appa knocked off some treetops on his ascent, which was later observed by Azula. The girls ultimately ditched the tank train in favor of their mongoose dragons; Azula followed Aang's trail while Mai and Ty Lee pursued Appa.
By using the concept of their tanks and building upon it, the Fire Nation was able to make a vehicle powerful enough to go virtually anywhere and large enough to transport people as well as weapons. It can often be spotted from miles away because of the plumes of smoke and steam bellowing from its engines.
The tank train is the size of a regular train and consists of several cars. The first car is the work-horse, which houses the engine. The vehicle runs on firebending and coal. This first car also has a functional design; the sharp, armored plow on the front is used to batter a path through obstacles, and it prevents large objects from getting underneath the train and damaging the undercarriage. There are also two huge grappling hook cannons on either side of the car. The next two cars, which can be added and removed as needed, are for hauling things.