The structure is a gigantic Melon Tree, with a hollow trunk which stretches underground to a train system. All of the materials used underground are wooden planks or stairs, with a ladder leading two and from the tree. For the World Spawn stop, the leaves are covered by snow to match it with the biome. It used to be the main transport of Stampy's Lovely World.
During construction, Part of AmyLee33's house was tunneled through. Some slight adjustments were made, but were never seen; Stampy never showed us the houses again. Lots of basements etc. are still very close to the tree train.
The tree is used to transport Stampy from different areas of his Lovely World. There are three Tree Train stations to be exact: one outside his house, one outside his Funland, and one at the spawn of the world. These were the former locations of the stations of the Minecart Track. The spawn of the world and the Funland one both only go to his house. Once they are at their place, they destroy the minecart, put it in a chest and exit on the ladder. There is also a route up onto the top of the tree, which boasts a great viewing area of the world. Norman used to live here as well in the World Spawn melon tree, but he now lives in the Emergency Igloo.
It has an episode named after it ('Tree Train') which was Episode 122.
It replaces an older overland train system that Stampy used to use, which was unfortunately abolished due to the amount of space the tracks took up on land, and the general ugliness of the cobblestone stops.
When it was first built, Stampy has short tournaments with his helpers to see who could reach the Funland first, however, they were done before the update which increased the speed of minecarts.
It was one of those buildings built where stampy sped up the building.
Although Stampy has said he wanted to extend the Tree Train to the northern parts of his Funland no work has been done so far.
He never used it since Episode 347, but it was returned in Episode 395 and 479.