Since the 1960s, Paris has gradually built itself to have a 5-line regional rail network connecting the city and its suburbs, with more than a billion riders a year. Unfortunately, investment has been slow in the last 20 years; the fifth line, the RER E, is being extended to the west, but other problems are not being fixed through more investment. Some regional rail lines remain disconnected from the system, including one of the city’s six intercity rail terminals, Gare Montparnasse. While east-west capacity is being augmented through the RER E extension, north-south traffic is jammed and yet is not slated to receive any relief, despite past studies.
Taking everything together, this is what Paris needs to do to complete the conversion of all commuter rail in Ile-de-France to RER standards:
Full-size image can be seen here; warning: 71 MB.
Dashed lines are new tunnels to be built. Most of the dashed green line is the under-construction RER E extension from Saint-Lazare to La Défense and points west. The remainder, between Les Halles and Auber/Saint-Lazare, is a new tunnel that should be built, giving away the extension to the RER D instead. With a full line extended, the RER D could take over the entire SNCF-run part of the RER A while also continuing west to Mantes-la-Jolie as is planned for the RER E extension, so the RER A can gain the Transilien L branches to the southwest with a short curve from La Défense to Puteaux.
In addition, what is now the shared RER B and D tunnel between Gare du Nord and Les Halles should be four-tracked; the stations at both ends thankfully already have separate platform tracks for the RER B and D, and in 2003 a somewhat disruptive plan to four-track the tunnel was estimated to cost €700 million. Since the RER D tracks are to continue west, the new dual track tunnel should continue south across the river and connect to Montparnasse, creating the RER F; the RER F should take over the current northern branches of the RER B to form a southwest-northeast line, while the current southern branches of the RER B should be connected to what are now the northern branch of the RER D and the branches of Transilien H.
The RER C and E should be broken and recombined using a short four-track tunnel across the river, creating northeast-southwest and northwest-southeast trunk lines. Today, the RER C misses the Paris CBD and has an awkward connection to the RER A; with this recombination, the connections would still require a lot of walking at transfer stations but they’d exist and passengers would get solid two-seat rides.
Finally, a handful of outer-urban and suburban fixes would be useful: a few infill stations, depicted with gray filling; using all four tracks on the RER F trunk line to Aulnay (currently the RER B) to make it easier to run express trains to Charles-de-Gaulle; building a short suburban tunnel through Chaville to connect the RER F and A branch; continuing T3 to form a full circle using the Petite Ceinture in lieu of the awkward RER C branch today; constructing an infill station at the RER E/F junction in Meudon.
Excluding the ongoing RER E extension, the total length of new tunnel in city center is 8 km of two-track tunnel and about 1.5 km of four-track tunnel. This would set non-Anglosphere world records in construction costs per kilometer, just as the RER A did; costs in the €400-500 million per km can be expected given the complexity of tunneling under so many older Métro lines, so the system would cost around €5 billion, perhaps reaching €6-7 billion with the extra suburban tunnels and infill stations.
The map doesn’t go to the edge of Ile-de-France, or else it would be even bigger, but the plan should be to connect every Transilien line to this system, even ones in faraway exurbs. Frequency to the exurbs need not be very high – today they get hourly service off-peak, and half-hourly service in the future should be plenty to small towns on the edge of Ile-de-France; of course, closer-in suburbs as well as major secondary centers like Meaux and Evry should get much higher frequency, and the trunks should get a train every 3-4 minutes even off-peak.
The point of this exercise is that Paris has already done the hardest parts. The RER A and B exist, and Châtelet-Les Halles was dug at enormous expense in the 1970s. Even at the high per-km costs of connections underneath the center of Paris, the tunnels Paris needs to build in the next 10-15 years are low-hanging fruits for completing the project of connecting the entire region through a unified RER network.