From Nookipedia, the Animal Crossing wiki
The train station is a location in Dōbutsu no Mori, Animal Crossing and Animal Crossing: New Leaf. The train station is run by Porter, a monkey. In Animal Crossing: Wild World and Animal Crossing: City Folk, the train station was replaced by the town gate.
The train station is located at the top of the player's town, and can be used to travel to other towns. On Saturday nights, K.K. Slider can be found sitting on the stool outside the train station from 8:00 PM to 12:00 AM. The player arrives via train at the start of a new game and exits at the train station.
If the player has a second memory card, they may use the station to visit other towns using one of two methods. If the second Memory Card has Animal Crossing save data, the player will travel directly to the other town. If the second Memory Card does not have Animal Crossing save data, but does have at least three free blocks, the player can use it to copy their data, such as their appearance, Bells, and items, in order to travel to a town on a third Memory Card. The player's data will be saved to the second card, which can be inserted into a GameCube with a Memory Card with Animal Crossing save data; next, the player will enter the town on the third card. To return to their home town, the player inserts the Memory Cards containing the home town and the player's character data into a GameCube.
Animal Crossing: New Leaf
The train station makes a comeback in Animal Crossing: New Leaf where it is once again run by Porter. It replaces the Town Gate at the top of the town. As in the original Animal Crossing, the player enters the town through the station at the start of the game. At the station, the player can open the town's ticket gate to allow guests from other towns to visit over local wireless or WiFi. While any player can visit over local wireless, only the player's 3DS friends can visit over Wi-Fi. The player can travel to other towns from the station, as well.
The train station contains a locker, which the player can use to access their stored items. Additionally, when the town's mayor begins a public works project that will be constructed on Main Street, Lloid will take donations at the station.
If the player has been visited by other players at least 100 times, they may remodel the train station as a public works project. The remodeling options are as follows: