In three cities that I know of, there are plans to deal with an incipient regional rail capacity crunch by building a new tunnel: Tel Aviv, Hamburg, London. The route in question in all cities already has regional rail service making frequent urban stops as well as longer-distance intercity trains. Setting Tel Aviv aside – new tracks are not necessary there at all – both Hamburg and London have a choice of what to build in the tunnel. In both cases, the answer must be intercity rail and not regional rail. This affects Crossrail 2 in London, currently shelved but still in active planning, as well as plans for Hamburg Hauptbahnhof-Altona capacity improvements.
The dominant factor in the cost of an expensive urban railway in a constrained environment is the stations. Low-cost countries build very cheap stations, but that’s true in outlying areas, urban as they may be, and not in city center areas under and around older subways. What’s more, Britain and Germany are not low-cost countries. German costs are somewhat higher than the global median, British costs among the world’s highest. Thus, keeping down station costs is paramount – and express tunnels have fewer stations than local tunnels.
Normally, the express vs. local issue is not relevant to a new urban rail line. Yes, more stations are more expensive, but on a line designed to open up service to a new area, more stations also provide more access, so the extra cost is often worth it. This is true even for urban subways that act as relief lines, like Second Avenue Subway, a relief line for the Lexington Line: more stations provide better local access and therefore increase the line’s relief value.
However, when the problem comes from regional or longer-distance capacity, all of this goes out the door. Crossrail 2 includes a long tunnel from Central London all the way to Wimbledon not because of purely local needs but because of very high rail usage along the South West Main Line. In Hamburg the problem is similarly about the main line between Hauptbahnhof and Altona – local traffic is saturated on the S-Bahn, and all other trains have to squeeze on the remaining two tracks of the Verbindungsbahn. Thus, capacity expansion should involve a tunnel with the fewest number of stations, on the most express services.
And yet both cities are doing it wrong. Hamburg is planning an S-Bahn tunnel, with the existing S-Bahn route then given over to regional trains, to be segregated away from the intercity trains. But there are already two Hauptbahnhof-Altona S-Bahn routes – that’s not where the service need is. Instead, new tunnels should go between the stations without stopping, to reduce costs, hosting intercity trains while regional trains take over the existing intercity tracks.
London is likewise planning on an undulating connection between Clapham Junction and Wimbledon, with links to other parallel north-south lines in the area. This is not good planning – those new stations are inordinately expensive and not needed for network connectivity. If there is no way to six-track the South West Main Line above-ground by replacing the sloped berm with retaining walls, it’s the fastest trains that should go underground, to save money on stations. Crossrail 2 is a £31.2 billion project; I don’t think Paris has spent this money on all Métro and RER lines in the region to date combined, and Grand Paris Express, at a broadly similar cost, includes 160 km of tunnel. It’s necessary to economize and build the tunnels that are necessary, and not the ones London would like to have.