Linebeck III is the grandson of Linebeck from Phantom Hourglass, but he himself is in Spirit Tracks. Because of the suffix at the end of Linebeck III's name, it is implied that his father was Linebeck II, but he was never actually mentioned. Linebeck III lives in a small shack in a station called the Trading Post in the far right corner of the Forest Realm where he owns his own store, the Linebeck Trading Company. Much like his ancestor was, he is obsessed with treasure and will buy any extras that Link is willing to sell to him. He also might give Link parts for the Spirit Train instead of Rupees. Also, if Link loses his shield to a Like Like or otherwise, he will sell Link a new one for the (overpriced) value of 100 Rupees.
Linebeck III is first met when Link and Princess Zelda attempt to journey across the bridge and into the Ocean Realm. However, when they get there, they realize that the bridge is ruined and un-passable. They stop their train at the closest station, which happens to be the Trading Post. When they ask Linebeck III for help, he suggests the bridge builder that lives in the Snow Realm. Link and Zelda travel up there, and the worker agrees to help rebuild the bridge. They bring him all the way back to the Trading Post, and he repairs the bridge. When it comes time for the bridge worker to receive his pay, he reveals that Linebeck III owes him money from the time that he did work on his house, totaling the bill to a whopping five thousand Rupees. Linebeck III panics and dumps the entire pay all on Link, then runs back into his house. Afterwards, however, he asks for Link's help to pay back his debt. He gives Link a letter with instructions to find a hidden Regal Ring that his grandfather, the original Linebeck, had left for him if he ever needed it. The Regal Ring is priced at 8000 Rupees, more than enough to pay back the worker. The only problem with getting the ring is that Linebeck III doesn't know how to decipher the riddle that his grandfather left him. Link takes the letter and solves the puzzle, then brings the ring back to Linebeck III and the worker. The bridge worker takes the ring and leaves, leaving Linebeck III speechless. After Link completes this quest, Linebeck III will buy any of Link's treasure and sell him train parts.
Later in the game, Linebeck III can also be used to collect one of the game's many Force Gems. After Link obtains the Freight Car from the Gorons, he can use it to bring Dark Ore to Linebeck III. The Dark Ore can be found at Dark Ore Mine, but spoils quickly when it hits sunlight. If Link wisely uses caves and Archway Gates to quickly bring at least five parts of Dark Ore to Linebeck III, he will reward him with a Force Gem. This Force Gem allows Spirit Tracks to appear in the Southeast area of the Ocean Realm, creating access to the Lost at Sea Station.