Lucid Time!

Considering I went to bed past midnight, lucky I had recall at all.

Fragment: Something to do with cat vomit. A lot of it. I had to clean it up.

Fragment: Something to do with a game where you built and fought with Lego airplanes. I was helping other people refine their designs.

So I was watching some series that was supposedly a Spin-Off from Korra. I think it actually took place even later on the timeline, because the technology in here was very advanced. In Legend of Korra, they love to show off the new technologies as the world advances and this was no exception.
The story starts off with Ryu, the main character living in an Earth Kingdom village. It turns out that he uses his earthbending to make sculptures, but isn't very good at it.
Then one day, the village comes under attack by some common bandits and he tries to fight them off and give chase. He chases them onto a train and the train leaves before Ryu can fight them off.
After defeating all of the bandits, Ryu is thanked by the original owners of the train. He learns that the train was actually a hijacked military train and they were looking for a group of criminals called the Mao gang.
...
Some time passed. Ryu is taught how to Metalbend by the military people and (despite his art skills lacking) he is a good fighter.
...
Then I recall there was a train chase. The Mao's train has a high-speed engine and begins catching up to them. There are some Firebenders and Airbenders who fire on the back of the train. Earth and Metalbenders from the military train retaliate. They fire a grapple into the engine of the Mao's train and slide across. But the boarding party is promptly defeated.
Mao's train then reveals it has a secret. One of the cars' roofs opens and out come helicopters. Ryu is on the roof just in time to see this new technology. For some reason (Out of character for the show) the choppers have machine guns on them.
Ryu's commander uses Metalbending to stop the bullets, and tells Ryu to go. Ryu tears up part of the train with Metalbending and makes a glider to fly across. The commander continues fighting the helicopter.
Ryu fights his way onto the enemy train. He manages to make it to Mao's war room, near the back of the train. Here it is revealed that there is no 'Mao Gang' but that Mao is just one person, with a multiple personality disorder. He is bald, and I am speculating that he might be an Airbender.
He has a large map, and for some reason the map is of Earth, not the avatar world. He has 'X's over the areas that he wants to conquer.