Stif has given RATP the go-ahead to proceed with the first technical studies for the project, which is expected to cost around €256m, including a €100m contribution from Stif.

RATP notes that the conversion of Line 1 to Unattended Train Operation (UTO) has enabled operating headways to be reduced at peak times, improved reliability, and allowed service patterns to be adapted to short-term fluctuations in demand. According to Stif, average peak punctuality on Line 1 reached 98.7% in early 2013, compared with 95.8% for the overall metro network.

Line 4 will receive the fleet of MP89 trains currently in use on Line 14, which will be released by the delivery of new trains being ordered in connection with the extension from St Lazare to Mairie de St Ouen.

The 12.1km Line 4 is the second-busiest metro line in Paris, carrying around 740,000 passengers per day. Patronage has increased recently following the opening of the 1.5km southern extension from Porte de Orléans to Mairie de Montrouge, and a further 1.7km extension to Bagneux is due to open by the end of 2019.