Since World War Two, some cities have sought to extend rapid transit into their suburbs by leveraging legacy commuter rail lines. Building on prewar examples from
Berlin and
Tokyo, they initiated a variety of treatments to modernise their commuter rail: electrification, integrated fares, high all-day frequency, and cross-city connections.
All this turns commuter rail into an express metro line. The city that has done the most in this direction is Paris, which since the 1970s has built a network called the RER, with five lines labeled A through E.
It is the cross-city connections that are the costliest to provide, since they almost always involve new tunnels under city center. Cities can build cross-city tunnels in two ways. One approach involves high investment: the tunnels are longer and involve several stations, often in difficult-to-construct locations. The main example is the RER A, whose construction involved about 17 km of new tunnel and seven underground stations, running on an east-west axis through central Paris.
The other approach is lower-investment: tunnels are the shortest possible connecting commuter rail terminals. The main example is the RER C, whose construction involved just 1 km of new tunnel and no new stations, creating an southeast-to-southwest line on the Left Bank of Paris.
This is a spectrum rather than a binary division: RER lines B, D, and E are intermediate between the high investment that went into the RER A and the low investment into the RER C. In layout, the RER B is quite similar to the RER A, but managed to leverage a legacy line reaching within 2 km of city center.
The same division between the two approaches holds outside Paris, too. In London, Thameslink is similar to the RER C, whereas Crossrail, with its long new tunnels, is like the RER A, as is the planned Crossrail 2. Berlin's
North-South Tunnel from the 1930s, creating a new axis in the city complementing the older east-west Stadtbahn, is like the RER A.
North American projects, including the SEPTA Regional Rail tunnel in Philadelphia and the ongoing Toronto RER project, are both like the RER C. The Regional Rail tunnel connected two commuter rail terminals to create a mainline the shape of an inverted L, with some lines self-intersecting. Toronto is fortunate enough not to need new tunnels at all, since all commuter lines serve Union Station, some coming from the east and some from the west.
The main benefit of the RER C style is that it is much cheaper. It involves less tunneling, and the city can choose to build fewer stations. When tunneling deep underground, the stations are the most expensive element: for example, in New York's
Second Avenue Subway, built deep to avoid street disruption, the tunnels cost $415m whereas the three new stations cost $2.2bn total. The central segment of the RER A cost about
5bn francs, corresponding to about €600m per kilometer in 2016 prices; no other
rail tunnel in the world has cost so much except some New York lines and Crossrail. Crossrail, the other major modern example of this type of construction, is even costlier, perhaps £750m per kilometer.
The main benefit of the RER A style is that it lets commuter rail act as an express metro line. Such tunnels do not follow the shortest path between legacy terminals: both the RER A and Crossrail were designed as express east-west lines through city center, with stations connecting to most intersecting Metro or Underground lines. And they are not just commuter rail schemes but also relief lines for the busiest metro lines, namely Metro Line 1 and the Central line. RER C-style lines do not necessarily provide this: the RER C is parallel to Metro Line 10, the least busy in Paris.
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The biggest cities should probably plan on at least one RER A-style commuter line. London came to this conclusion when it began the Crossrail program; despite the high cost, it is now very likely to build Crossrail 2. The largest North American cities should learn from this and consider some truly metro-like commuter lines rather than just lines in the mold of the RER C.
