It has existed in its current form since 4 April 2009, when former line 1B, which ran between Stockel/Stokkel and Erasme/Erasmus, was shortened to Gare de l'Ouest/Weststation.

The section between Gare de l'Ouest and Erasme is now served by line 5.

The line crosses the municipalities of Molenbeek-Saint-Jean, Koekelberg, City of Brussels, Etterbeek, Woluwe-Saint-Pierre and Woluwe-Saint-Lambert.

The first section of this line was built in the late 1960s between Schuman and De Brouckère, but was served by trams.

[2] Line 1B was later expanded westwards, to Sainte-Catherine/Sint-Katelijne in 1977, to Beekkant in 1981, to Saint-Guidon/Sint-Guido in 1982, to Veeweyde/Veeweide in 1985, to Bizet in 1992 and finally to Erasme in 2003.