These plans were initially given some positive attention by the policy makers of Prussia and the Duchy of Nassau, but were rejected in 1853 and a licence was issued to build the East Rhine Railway instead.

In 1861, several local committees in Prussia and Nassau, published a publication supporting the construction of a "Main-Lahn-Sieg Railway", connecting Frankfurt and Cologne via the shortest route and also contributing to the development of mineral deposits (such as basalt and clay) in this area.

On 17 February 1868 the Prussian parliament passed a law that among other things, authorised the construction of a line from Limburg to Hadamar.

In 1872, it had been decided to build the Main-Lahn Railway (Main-Lahn-Bahn) from Frankfurt to Limburg via Idstein, Camberg and Eschhofen.

In May 1879, the Royal Railway Division (KED) of Wiesbaden received permission to continue the construction.

Between the summer of 1977 and the winter of 1991 another pair of express trains also operated between Altenkirchen and Mainz and Wiesbaden.

After the privatisation of Deutsche Bahn on 1 January 1994, operations on the line between Limburg and Au were taken over by DB Regio AG.

The Hessische Landesbahn (HLB) was awarded the contract for the operation of the network from August 2015 until December 2030, including the RB 90 service (Westerwald-Sieg-Bahn).

Due to delays in planning, infrastructure measures could not be completed in time for the timetable change in December 2015.

In the northern part, which is the responsibility of SPNV Nord and NWL, the trains already ran hourly to Siegen, as provided for in the new timetable concept.