From the City Journal "Brisbane-Capitol-Stanthorpe Megaregion"

In 2012, the Brisbane State Government announced a new infrastructure project to relieve pressure on the city's inner rail network. The $6b project, named the Brisbane Underground Rail Project (BURP), would augment the commuter rail network around Brisbane Central station. The announcement was in reaction to decades of congestion on the antiquated C-Train Network. Prior to the BURP, all train services on the C1, 2, 3, 4, 5, 7, 8 and 9 lines were forced to terminate at one of Brisbane Central's eight platforms,. The consequence was that trains would be forced to dwell longer at the station, limiting the frequency. Stop-gap measures such as a driver at each end of the train allowed trains to leave quickly, however this meant less flexibility with staffing and limited the frequency on these lines. Under the BURP, trains would dive into new tunnel portals from the north and swing underneath Brisbane's central business district, arriving at new platforms 40 metres beneath Brisbane Central. Trains would then move eastward and surface just before East Brisbane station, joining the existing line running toward Petrie and Stanthorpe. The consequence of this is the vast majority of trains can be throughput, without terminating, vastly increasing the capacity. It would also de-clutter the network from eight lines to just five. The existing station's platforms would remain operational for the C4 and C5 lines, as well as terminating services from the east. The northern portal required the demolition of several properties and the permanent closure of Fysh Rd and Smith St. Demolition began in 2013 and finished the next year, before digging could begin. For a period of six weeks, tracks in both directions were closed as work was done to connect the new tracks to the existing. Finally, the finished product: On the eastern end, the work took place entirely within the existing rail corridor. While it required no resumptions of nearby properties, disruption was immense. Buses replaced trains during intense work period, stretching between 2014 and 2018. The finished product: The project opened after extensive testing in October 2019. Services altered: The C1 and C9 lines were merged into the new C1. During peak, some C1 trains to/from Stanthorpe still call at the old above-ground platforms. The C2 and C7 lines were merged into the new C2. The C3 and C8 lines were merged into the new C3. The C4 and C5 lines remain the same, continuing to terminate and turn back at the existing above-ground platforms. Thanks for reading!