At its meeting of July 25, 2013, the Sound Transit Board approved a pair of contracts for engineering studies to design a turn-back track at International District Station (IDS) for East Link trains going in and out of service at the SODO Operations & Maintenance Facility (O&MF).
The background for the engineering study contracts state:
An interdisciplinary Sound Transit team agreed that a turn-back track facility along the Central Link alignment would improve train movements between East Link and the OMF. These improvements include the ability to add or remove light rail vehicles for early morning start up and peak period demand as well as the ability to accommodate a disabled train. Various options were evaluated for cost, construction impacts, service impacts, and operational efficiency. The option of a turn-back track facility at IDS was selected.
Sound Transit staff declined to elaborate on whether a turn-back track at one of the other stations was considered as part of this process.
Since the contracts were merely for engineering studies, and much larger construction contracts have yet to be approved for this work, there is still time to take a look at the alternatives that allow for a center platform at IDS instead, and put the turn-back track(s) in Pioneer Square Station (PSS), University Street Station (USS), or Westlake Station (WS).
Another group of options would be to have a crossover track from the northbound platform of one or more of these stations to the southbound exit at that station. That is to say, East Link trains going out of service would pull up along the platform, and then turn back along a crossover track at the south end of the station splitting from the northbound track about 20-30 feet north of the south end of the station, merging with the southbound track a few feet north of the south end of the station. In order to avoid tracks crossing over each other, no station could simultaneously have both a center turn-back track and a crossover track.
It bears repeating that the center platform option we’ve been talking about in IDS is to add a center platform, not to remove the outer platforms, which would enable use of all 32 train doors while at IDS, in a variation of the “Spanish Solution” (in which passengers enter the train on one side and exit to a separate platform on the other).
In the comparison and analysis below, I will focus on the following aspects: operational safety, operational cost, construction cost, trip time, and peak operational throughput.
Safety is Job One
Let me start out by saying I am no expert on these issues and have no professional credentials related to the topic. But I do have a degree in Mathematics.
One benefit of having a turn-back track in PSS, USS, or WS compared to IDS is the ability to get the big picture and minimize opportunities for derailments.
In the IDS turn-back track plan, there is a merge switch between East Link trains headed north and South Link trains headed north. There is a split switch for North Link trains to head either south or east. North of the merge, there is a split switch for northbound trains to shift onto the turnback track. There is a similar switch for trains turning back to merge with the southbound trains north of the southbound split. There is also a switch right in the middle where the northbound trains turning back merge, change direction, and then split to the track that gets them going southbound. The turn-back increases the number of switches station supervisors have to pay close attention to from two to five. On top of that, consider where the northbound split switch is proposed to be located: a mere several yards north of the northbound merge switch. Then, before the train turns back, two or three switches have to be flipped (depending on whether the train will then proceed east or south), all within close proximity. This should give some railroad safety experts at least a tinge of angst.
Moreover, the person in charge of the switches is responsible for the orientation of all five switches. If, instead, a center platform were built in IDS, there would only be two switches in the area south of the station, and they would not need to be coordinated with each other.
A turn-back track in PSS, USS, or WS would involve three switches, all coordinated with each other. A crossover track in one of these locations would involve only two switches.
So, you must be wondering which station has the longest distance to allow the greatest stopping distance for a train turning back. That would actually be University Street Station. The other stations, including IDS, are nearly identical in length, but about 20 feet shorter than USS. PSS has a few more feet than IDS in the crucial central area between the platforms. But then, you must be wondering why the area south of IDS isn’t being used to lengthen the turn-back track. Maybe it could be, but the diagram shown (above) to the ST Board shows that the switches will be at the entrance to the station.
In short, staff has made no clear case that the turn-back track is safer at IDS, and there are reasons to believe it is actually the most dangerous option.
Operational Cost – Much Ado About Almost Nothing
One of the reasons for placing the turn-back track in IDS is to minimize deadhead time for East Link trains going into and out of service at the SODO O&MF. So, let’s calculate the rough daily operational savings of having it there compared to having it in the middle of Pioneer Square Station.
Just to have an apples-to-apples model, lets assume each line (Everett-to-Redmond and Everett-to-Tacoma) is a 4-hour round trip for a given train, including layover time. (If it is longer or shorter, that won’t alter how the two options compare.) Let’s assume ten-minute off-peak headway and six-minute peak headway on East Link and South Link. Let’s also assume half of the trains stay at the SODO O&MF, and the other half are split among south, east, and north O&MFs.
To maintain 10-minute headway on the Everett/Redmond line, 24 trains would be required. Add a similar 24 trains on the Everett/Tacoma line, for a total of 48 off-peak trains. Decreasing headway to 6 minutes would increase the fleet requirement to 40 trains on each line, or a total of 80 trains. After morning peak, 32 trains would be brought out of service and later returned to service. This makes a total of 224 goings into service and comings out of service in a typical weekday. Approximately 112 would be to/from the SODO OM&F. Let’s further assume those trains at SODO are being distributed equally, with one half headed north or coming from the north, a quarter headed or coming from the south, and a quarter headed or coming from the east. That leaves approximately 33 times a day the turn-back track would be used. Ideally, it should be even less, as it makes more sense to add and subtract trains at the end of a line in order to control headway, but let’s go with 33. Having to go an extra station to PSS, and come back that same extra length, would add approximately four minutes under the current joint-operations speed limit, but will be less when joint operations cease, but let’s go with four minutes to be sporting.
Do the math, and we get two hours and twelve minutes that might be saved each day by putting the turn-back track in IDS instead of PSS.
But wait. We still have to calculate the cost of longer dwell times at IDS due to not implementing our proposed variant on the Spanish Solution. It should be plainly obvious that using 32 doors on every train instead of 16 will reduce dwell time. The question is by how much would that dwell time need to be reduced, on average, to overtake the deadhead savings. To get that number, we need a count on the number of in-service trains passing through IDS on a typical day. I’ll skip the details and tell you that, based on current peak, off-peak, and 15-minute headway periods, it would come out to 544 daily stops in IDS, or so. At 7 seconds of extra dwell time per stop in IDS, the deadhead savings gets erased.
On top of that, if the extra dwell time pushes operations to have one more train in service during either peak or off-peak in order to maintain headway, the additional platform hours for that train overwhelms the paltry savings on deadhead.
The cost of building the turn-back track should be virtually identical regardless of where it is built. The only additional cost to build it further north would be more money for engineering, similar in amount to the $955,000 in contracts the ST Board approved for engineering the turn-back at IDS.
The cost of building a center platform at IDS should be the cost of pouring concrete and installing the textured strips, plus adding an emergency elevator at either end (to comply with ADA). Design and engineering would be involved, of course.
Passenger Convenience & Trip Time
Passengers transferring between the east and south routes will spend a couple extra minutes getting up to street level and descending again, instead of stepping off the train, walking a few steps westward, and stepping onto the next train going the direction they want.
As the system grows, this transfer nuisance will impact close to one-sixth of all destination pairings in the system. Another third will be impacted by longer dwell times at IDS. The remaining half will be indirectly impacted during peak by increased minimum headway…
Peak Train Throughput Capacity
This is where the decision to place the turn-back track in the middle of IDS turns from safety cheap-out, minor daily operational waste, and down-prioritization of passenger convenience, into a major design flaw. IDS will probably be the busiest station in the system, due to east/south transfers. If it is, it will be the bottleneck that will dictate minimum headway. Any increase in peak dwell time at IDS will increase minimum peak headway on North Link by the same amount, and will increase minimum peak headway on East Link and South Link by twice that amount. This will affect the number of trains that can get through the tunnel during peak of peak. Add to that the time trains will spend sitting and waiting for the go-ahead signal to merge coming into the station, and the dream of 3-minute peak headway on North Link, 6-minute headway on East Link, and 6-minute headway on South Link may have just gone up in smoke.
If ST Staff has more reasons not to build a center platform at IDS, I hope they will offer them to the public. If there aren’t more reasons than already publicly presented, then the lack of a center platform at IDS may be the biggest design blunder in the history of the Link system.
It should be pointed out that this design flaw is not in how the stations were built in the first place. Indeed, the extra space in the middle of each station is what is enabling the options for where to build a center platform. One of the ST Board members tried to blame the Board’s decision to move forward with building a turn-back track instead of a platform in the middle of IDS on elected officials lacking foresight 20 years ago. No, the design flaw is how the stations are being rebuilt, and so totally on the shoulders of the current ST Board, if the Board opts to proceed with construction of the flawed design.
This blunder can be fixed now, at minimal cost, and in plenty of time for the 2019 tunnel closure and conversion to rail-only operation.