The Graz Köflacherbahn is the largest privately-owned public railway in Austria, running from the city of Graz to Köflach and to Wies-Eibiswald. It was one of the first railways in Austria, and was built to cater for the movement of coal. The Voitsberg-Köflach-Lankowitzer coal mining company needed a railway line to move the lignite it mined in the Köflach area. On 26 August 1855 the company received the concession to construct and operate a freight and passenger line between Köflach and Graz. Construction started in April 1857 after securing finance, but the war between the Austro-Hungarian Empire and Italy in 1859 hastened the construction of the line, to provide fuel for the army. The line opened for coal traffic on 22 June 1859, when the first train of eleven coal wagons departed from Köflach for Graz. The 40 km-long line was officially opened on 3 April 1860. Freight traffic travelled over a connection to the Südbahn at Graz Hbf., but passengers alighted at Graz GKB, to the south of the Hauptbahnhof, and remote from the city centre.

Lignite was found in the Steyeregg and Eibiswald area, to the south of the existing line, and the railway decided to build a branch line to the coal field. The concession was granted on 8 September 1871, and the line, 50.99 km long, opened from the junction at Leiboch to Weis on 9 April 1873. However, financial problems dogged the GKB for some years, and on 28 June 1878, the GKB decided to lease the track for fifty years to the k.k. priv. Südbahngesellschaft, and the lines were taken over from 1 September 1878. In the following years, several plans for expansion were drawn up, but none were started. However, in 1892, a narrow-gauge line was built by Steiermärkischen Landesbahnen from the station at Preding - Weiselsdorf to Stainz, the Stainzerbahn.

On 31 January 1906 a concession was granted to Sulmtalbahn AG to operate a line from Gleinstätten south-west for 4.66 km to the southern branch at Pölfing-Brunn. The goods line opened on 13 October 1907, trains being taken over on reversal at Pölfing-Brunn.

The after-effects of World War 1, and the liquidation of the k.k. priv. Südbahngesellschaft led to the line being absorbed by the BBÖ with effect from 1 January 1924. However little was done to improve the track or facilities, and the lines were slated for closure, until in 1928 the steel company Österreichischen Alpine-Montangesellschaft (ÖAM), who depended on the lignite supplies, intervened, and persuaded BBÖ sell a majority stake in the line to them. The company also took a majority share holding in the still-private Sulmtalbahn, and from 1 April 1930 the line was absorbed into the GKB. Proposals were made to connect the GKB with Karnten, and both goods and passenger services increased.

The parent company changed its' name in 1944, to "Steirische Bergbau- und Eisenbahngesellschaft" but was promptly absorbed into "Reichswerke AG für Erzbergbau und Hüttenwesen Hermann Göring" after the Anschluss. The company reverted to the old company name after the fall of the German Empire in 1945, when the share rights became the property of the State. ÖAM amalgamated with Vöest - Linz in 1974, becoming part of Vöest-Alpine AG, later part of Österreichischen Bergbau-Holding AG (ÖBAG). The transport division was constituted as a separate company, Graz-Köflacher Eisenbahn GmbH (GKE), from 1 January 1998, and the concession was extended until 31 December 2025. Since January 2004 the company has traded as Graz-Köflacher Bahn und Busbetrieb GmbH (GKB) - although it has traded as GKB on and off since opening.

As the supplies of lignite fell, and the heavy industries that used it disappeared, so the GKB has changed from being a mover of freight to be a prime passenger-carrier in the area south of Graz. Steam locomotive power ended in 1978. One engine, 671 (StEG Fabriks-Nr. 504/1860), which was built for the Südbahn and entered service in 1860, being transferred to work on the GKB in 1878, has never been withdrawn. Numbered 49.03 when in BBÖ service, and DRB 53.7111, the locomotive was used for filming and special events until 1970, the loco was put in the GKB loco shed at Graz. Worked on by Steirischen Eisenbahnfreunden (StEF) between 1975 and 1978, the locomotive was put back into service and is still at work today on special trains.

The GKB ceased hauling coal in 1998, but this loss has been replaced by general cargo handling, both on its' own network, and across Central Europe, as a result of its' subsidiary, LTE Logistik- und Transport GmbH, jointly owned by GKB and Porr Infrastruktur GmbH. This company runs daily freight services to Duisberg, Germany, and also on the Burghausen ( Bay ) - Mühldorf - Simbach/ Inn - Marchegg - Bratislava - Ziar nad Hronom (SK). Other services include methanol between Passau and Krems/Donau; ammonia between Passau and Linz; diesel between Passau and St. Valentin and LPG between Kufstein and Brenner. Cement is also hauled on routes between Germany and Austria, and also moves new and repaired locomotives and trains for a number of companies.

GKB also transports goods from 32 sidings and branches on its' 91.25 route-miles, the largest of which is wood sent out by Firma Holzindustrie Preding.

In 2002 GKB joined forces with local authorities at Lavamünd and St. Paul to help reopen the branch line to goods traffic again. The line reopened for business on 26 October 2002, using locos provided by the GKB