So, you have your urban rail line. It’s mostly above ground, so constructing new express overtakes is feasible. It has decent frequency, and carries trains to destinations at a variety of distances from city center. But it’s not an overcrowded subway line that brushes up against line capacity, requiring all trains to run at the same speed. Do you run express trains?
I’m going to focus on regional rail in this post, since with two Tokyo-area exceptions, proper subways are incapable of running express trains without dedicated express tracks due to their high frequency. On a line with a train every 10 minutes it’s feasible to mix trains of different speeds with timed overtakes; on a line with a train every 2 minutes, it’s not. I’m going to use the LIRR and Caltrain as examples, and then apply the derived general principles to other cases in the US, including future regional rail schemes.
The basic tradeoff of express service is that it provides faster service to the express stations at the cost of frequency at the local ones. This can be done in two ways: expresses that stop once every few stations, and local-then-express patterns. Jarrett Walker calls this limited versus express, based on bus service patterns; with trains, both types are called express. The subway in New York, the Chuo Rapid Line, Seoul Subway Line 1, and Caltrain baby bullets are examples of the first kind; the Caltrain limited-stop trains and the peak-hour trains on some LIRR lines are examples of the second kind.
Express trains of either kind but especially the first reduce line capacity, even with very long overtake segments. If train X overtakes train L, then there needs to be an available slot ahead of train L, and after the overtake there’s a slot opening up behind L. The Chuo Rapid Line runs a mixture of local (“rapid”) and express (“special rapid”) trains for most of the day, but at rush hour, there are only local trains, peaking at 28 trains per hour; on the shoulders of rush hour, there are some express trains, with total traffic of about 20 tph. The LIRR runs 23 tph on the Main Line at the peak, so this is an issue, which the LIRR unsatisfyingly resolves by running trains one-way at rush hour. It’s less an issue on Caltrain given constructable overtake locations, but right now the overtake locations are inconvenient and the trains are pulled by diesel locomotives, increasing the stop penalty and reducing the capacity of a mixed local-express line.
The second kind of express service is bad industry practice and should not be used. It avoids the capacity problems of the first kind at low traffic levels, but at high traffic levels the speed difference is still too large. It is used when the trains are a special CBD shuttle and makes it impossible to serve passengers who are cheap to serve, i.e. those getting off short of city center. Caltrain’s limited-stop trains do this because of capacity problems during rush hour, when they need to get out of the baby bullets’ way. The LIRR does this because of a cultural belief that trains exist only to shuttle people from Long Island to Manhattan and back; due to the same belief, it runs trains one-way at rush hour rather than giving up on rush hour express runs as JR East does.
The first kind of express service may or may not be warranted. It depends on the following questions:
1. What is the line’s expected traffic level? Low traffic, up to about 4 tph for a regional line, favors an all-local configuration to prevent cutting local stations’ frequency unacceptably. Very high traffic favors all-local configuration for capacity reasons, or else investment into long overtakes or even full four-tracking. Intermediate traffic, in the 6-12 tph range, is the best zone for express trains.
2. Have local trains already been sped up by use of good industry practices? Level boarding, high-acceleration EMUs, better track maintenance allowing higher speeds between stations, good timetable adherence allowing less schedule padding, and infrastructure preventing delays on one train from cascading to others allowing even less padding can all significantly reduce the speed difference between local and express trains. In some extreme cases, a local train can end up not much slower than an express train hauled by a diesel locomotive.
3. How long is the line, and how many stations does it have? Longer lines and shorter interstations both favor express trains, all else being equal. Intercity rail, which also has higher stop penalties because of the higher line speed, deserves more than one stopping pattern even at low frequencies.
4. How big is the difference between minor and major stations? It is crucial not to confuse current ridership with ridership potential, since lines with express service often pick winners and losers, after which the better-served express stations steal riders who live closer to bypassed minor stops. This is common on Caltrain, where some but not all express stops are major job centers.
5. Can intercity trains plausible substitute for express service?
It is question 4 that makes the difference in many cases. On the LIRR, the Main Line has a clear distinction between major stops (Mineola, Hicksville) and minor ones (all the rest). The Montauk Line does not. Note the ridership levels of the stations, going eastward from Jamaica to the end of electrification:
Queens Village: 791
Floral Park: 1495.5
New Hyde Park: 1725.5
Merillon Avenue: 766.5
Carle Place: 386
Cold Spring Harbor: 2083
Deer Park: 2708.5
Central Islip: 1787
St. Albans: 93.5
Rockville Centre: 3425
Massapequa Park: 1672.5
There are three ends of electrification: Babylon, Huntington, and Ronkonkoma. All have markedly more ridership than nearby stations, especially Ronkonkoma, though in all cases it’s an artifact of their being the ends of electrification, with many people driving in from farther east. Ronkonkoma has nothing nearby that justifies its ridership level, the highest of any suburban LIRR station; it’s a park-and-ride that has a lot of ridership because it’s the end of electrification and has express service.
In contrast, in Mineola and Hicksville, there really is a concentration of activity justifying their status. Both have trivial transit usage as job centers, but there’s enough of a core, especially around Mineola, to justify higher service, and Hicksville is also the junction of the Main Line with the Port Jefferson Branch: see the census bureau’s OnTheMap tool.
But there are no special stations on the Montauk Line. Excluding St. Albans, which is in New York itself and has to compete with cheaper and more frequent if slower bus-to-subway options, the ratio between the busiest and least busy stations is 2.4:1. A similarly flat situation occurs east of Hicksville, excluding the two end-of-electrification stations.
What this means is that the LIRR should only run local trains on the Babylon Branch and east of Hicksville, while maintaining express service on the Main Line west of Hicksville when there’s enough capacity for it. A similar analysis of other lines in the New York area should give the following answers:
Hempstead, West Hempstead, Long Beach, and Far Rockaway Branches: all local due to short length.
Port Washington Branch: probably all local due to short length, but if additional local stations are added in Queens, then some express trains to Great Neck may be warranted.
New Haven Line: very long, sharp distinction between major and minor stops all the way but especially west of Stamford, high frequency, four tracks give enough capacity for everything. The current configuration of nonstop trains to Stamford continuing as local to New Haven and local trains turning at Stamford is fine, except that the express trains should also stop at New Rochelle (a junction with the Hell Gate Line, which deserves service, but also a major stop in and of itself, with the third highest weekday ridership of Metro-North’s suburban stations) and maybe also Greenwich; HSR overtake considerations may require stopping also at Rye and Port Chester.
Harlem Line: generally favors local trains, except that White Plains is a major job center and thus a far more important stop than all others, independently of its better service. There are four tracks south of Wakefield, favoring express trains, but conversely charging subway fares and allowing free transfers to the subway would lead to a ridership spike as people switch from the overcrowded 4 and 5 trains. There’s a big dropoff in ridership north of North White Plains, so the current configuration of locals that turn at North White Plains and expresses that go nonstop south of White Plains is fine, as long as off-peak frequency is raised.
Hudson Line: favors express trains because of length and four-tracking. Although on paper there are more and less important stations, this is an artifact of service patterns. The secondary stations in Yonkers serve higher density than the busier stations in the proper suburbs, and the dense parts near Tarrytown are actually in Sleepy Hollow, about equidistant from the Tarrytown and Philipse Manor stations: see the New York Times’ population density map.
Erie Lines and West Shore Line: probably all local since the population density thins too uniformly going north, with Paterson as the major exception. There are somewhat denser anchors at the outer ends of some lines – Spring Valley and Nyack – but Harlem Line-style nonstops run against a capacity problem, coming from the fact that this part of the network is necessarily highly branched.
Rest of New Jersey Transit: the main lines (Northeast Corridor, Morristown) are very long and have some distinguished suburban job and population centers (Metropark, New Brunswick Morristown) deserving express service, but the branches (North Jersey Coast, Montclair, Gladstone) do not. However, the fare structure and off-peak frequency lead to much less ridership on the inner-urban segments in Newark, Orange, etc., than would be expected based on population density. In addition, the difference between major and minor stops is fairly small on all lines when taking electrification into account, sometimes as small as on the Babylon Branch: see ridership data per line and per station.
Although my initial decision in my regional rail plan to pair the Erie lines with the Atlantic and Babylon Branches of the LIRR was aesthetic, creating a northwest-to-southeast line, in reality the systems are fairly similar in their characteristics. More or less the same can be said about the Staten Island-Harlem system. There are no direct connections to intercity rail except at Jamaica and in the Metro-North tunnel to Grand Central, the lines pass through urban or dense-suburban areas, the interstations are fairly short, and there’s relatively little distinction between major and minor stops. (White Plains is the major exception, and Paterson is a secondary one.) This makes the Lower Manhattan-based system much more RER-like than the Penn Station-based one, which is longer-distance and practically intercity at places.
Finally, the same set of questions in the other three major Northeastern cities generally lead to the conclusion that no express trains are needed.
In Boston, there’s too little difference between major and minor stops on each line (see PDF-page 70) – somewhat more than on the Babylon Branch, but much less than on the LIRR Main Line. The most prominent major station is Salem, but the low-ridership stations farther in on the Rockport/Newburyport Line are in working-class suburbs; the ridership there is depressed because of fare and schedule issues coming from competition with buses, and good regional rail would get much more additional ridership from Lynn and Chelsea than from Salem and the suburbs farther out.
In Washington, current traffic demand is so low that express service would seriously eat away at the frequency offered to local stations. MARC and VRE ridership is so low that any analysis of travel demand has to start from geographic and demographic information rather than from preexisting ridership; the only major outlying destination on any of the lines is Baltimore, which can be connected to Washington by intercity rail, and which conversely has much less Washington-bound commuter traffic than the Washington suburbs. The closest thing to justifiable express service is that when the commuter lines closely parallel Metro, they should have wider stop spacing.
In Philadelphia, on most lines, express service eats away at frequency too much. The one exception is the PRR Main Line, with the SEPTA Main Line a possibility. Many lines have sharp differences between local and express stations: for example, Cornwells Heights on the Trenton Line is much busier than the rest. But a combination of low frequency and lack of easy overtakes (on the Trenton Line, the inner tracks should be mainly used by intercity trains, with only the occasional regional rail overtake if required) makes this not useful. The PRR Main Line actually has less difference between major and minor stops than many others, but it is longer and has short interstations and higher frequency. The SEPTA Main Line has the frequency to support multiple stopping patterns, though the population density near the minor stations is high and the problem, as in other Northeastern cities, is high fares and lack of integration with urban transit.