The longest model railroad is the Loxx model in Berlin. According to this article:

Berlin in Miniature

It is operated by 34 separate computers running Fun for families in Berlin photo400 model trains on over 4 kilometers of track winding across bridges, past factories, and apartment lined neighborhoods through road crossings where 10,000 cars and 50,000 miniature inhabitants go about their shrunken daily lives (a miniature day lasts forty minutes, apparently). The control console takes up a good portion of a wall with video monitors tracking all the running trains, automobile traffic and city operations. You can follow the S-Bahns, Regional rail, and Ice Trains as they make their way around the city and environs.

The model covers 3,000 square meters (about 3/4 acres).

[See record #42]