From the Super Mario Wiki
The Training Room is a location in Luigi's Mansion. It is one of the places Luigi can go from The Lab, along with the Mansion, the Gallery, and the Hidden Mansion. In this room, Professor E. Gadd sends Gold Ghosts in for Luigi to fight, starting with one and ending with ten. E. Gadd congratulates the player depending on how many ghosts they have captured. If the player captures seven or more, fireworks explode and Professor E. Gadd happily congratulates Luigi.
The Nintendo 3DS version features a slightly different Training Room and Gold Ghosts pop out of the warp pipes in an orb-like shape, similar to the Gold Ghost carrying a key in the Foyer.