There’s a discussion going on at Greater Greater Washington about future expansions of the Washington Metro, adding more coverage and capacity; read both the posts and the comments, because there are great debates about just how much concrete really is needed. The post itself mentions various possibilities Metro has been looking into, a few good and many really awful.
Part of it is that the nature of such discussions favors concrete – it’s much easier to discuss a fantasy map than schedules and organization. Indeed, on my three regional rail posts on The Transport Politic, most comments concerned the proposed through-routing map and infrastructure to be built rather than schedule integration. The reason the comments on the GGW post are so good is that many eschew this and instead talk about other things. Even the idea of separating the Blue Line from the Orange Line in the city, which looks sound to me and is not yet another outbound extension to the exurbs, is suspect and there’s a serious suggestion to build light rail to relieve the capacity problem instead.
Discussed in the comments but not by Metro is the possibility of converting the commuter lines to rapid transit. Only one, the Penn Line running along the Northeast Corridor to and beyond Baltimore, is even electrified, and the rest are owned by CSX and Norfolk Southern. This would be far superior to adding more outbound Metro extensions, which have very high costs: the Dulles extension is $180 million per km despite being predominantly above ground.
The Washington Metro, and even more so BART, is more an S-Bahn or RER system than a subway. The stop spacing is very wide, and the lines branch out and go deep into suburbia. Unlike BART, Metro sometimes gets it right and has good transit-oriented development, though it too has its share of parking lot stations. The main difference is that due to poor organization (FRA regulations, pure agency inertia), the Washington Metro exclusively uses greenfield alignments, whereas S-Bahn and RER systems use predominantly existing commuter lines, with strategic tunnels built to provide service to the urban core.
There are two potential problems with relying on legacy commuter lines, aside from organizational difficulties that should be the first to be tackled. First, those lines may have capacity problems; this is not true for the Penn Line, but may be true for the other, lesser-used lines, because of freight conflicts. Second, the lines may not run to the desired destinations. Both concerns can be mitigated at much lower costs than pouring concrete on new lines.
First, the Penn Line is 64 km long from Baltimore to Washington, has 8 stations, and has no sharp curves except at the Baltimore and Potomac Tunnels, a $750 million replacement for which has already been studied. A 160 km/h train with the acceleration profile of the FLIRT and 30-second dwell times at intermediate stations (easy with level boarding) could do it in 36 minutes. Add Swiss-standard 7% schedule padding and about 30 seconds for slowdown through the B & P replacement and this is a total of 39 minutes end-to-end. A speed boost to 200 km/h would save maybe 2.5 minutes, since regional trains accelerate slowly at high speed. Local trains currently do the trip in an hour.
The Acela, a substandard train, is currently scheduled at 38 minutes, so all trains would travel at about the same speed, eliminating all capacity problems. (Current peak throughput is 7 tph, so the slight unevenness in the travel speed is a non-issue.) A mild Acela speedup, involving trains running at 200 km/h with no slowdowns, would speed the train to 22 minutes, and require just one mid-line overtake if peak traffic is to be 4 tph each for Acela and commuter trains. 6 tph each would require two overtakes and a lot of discipline, but would be doable given the capabilities of ERTMS. Full-fat high-speed rail in the Northeast would do the same trip in about 16.5 minutes, and require the line to be fully four-tracked; this is already part of MARC’s long-term plan. There is in other words no real problem with capacity as far as conflicts with intercity trains go.
Second, one often-overlooked point about S-Bahn/RER networks is that they have a fair amount of greenfield track, often in tunnel, constructed strategically to connect to important destinations off the existing rail network. For example, tunneled alignments bring regional trains to Charles de Gaulle and Zurich Airports. If there’s an important suburban destination not reached by Metro or a rapid transit system based on the five existing commuter lines, it should be fine to construct a spur – for example, the extension of Metro to Tysons Corner is a great idea. However, such spurs should be kept as short as possible, especially airport spurs, since airport connectors tend to underperform.
Look again now at the suburban lines proposed by Metro in the link. The Brown Line is a duplicate of the Red Line, which has no serious capacity issues except at its center. The Beltway Line skips the major centers that a circumferential should hit (for example, Arlington/Alexandria), defeating the entire purpose of a greenfield alignment. The outbound extensions would just create more transit-oriented sprawl, with people driving to stations and taking trains only at the peak. And it would all cost much more than electrifying track, purchasing good rolling stock, and running it with high schedule discipline.