According to Official Guinness Records,
The largest model train set has a total track length of 15,400 m (50,525 ft), and is located at the Minatur Wunderland (Germany) as verified in Hamburg, Germany, on 10 September 2015.Miniatur Wunderland (German for 'miniature wonderland') is a model railway attraction in Hamburg, Germany. The railway consists of a track in H0 scale, divided into nine sections: Harz, the fictitious city of Knuffingen, the Alps and Austria, Hamburg, America, Scandinavia, Switzerland, a replica of the Hamburg Airport and Italy.
For a complete list of 2015 records, please visit 2015 Guinness Records in Germany.