Keeping it lean and mean: getting the maximum ‘bang for buck’ from the CBD tunnel (Part 2)

This post is part two of a collection of affordable and effective ways to get the most out of the proposed CBD tunnel, see part 1 here.

5) Modify the Britomart throat crossovers to maintaining the ability to terminate long distance trains.

One of my greatest concerns about extending Britomart into a CBD tunnel is that is compromises the ability to terminate intercity trains at Britomart. The problem is that even once a tunnel is built out the western end of the station and up through town, all trains will still have to vie for space through the existing eastern tunnel. The problem is compounded by the fact that the platforms at Britomart face the eastern tunnel, and of course that diesel powered trains could not run through the new CBD link. So any diesel powered intercity train will have to enter Britomart from the east and exit out to the east again, which means each terminating train passes over the crossovers at the head of the platforms not once, but twice. Obviously this would be an issue if we are trying to run sixty suburban trains an hour through the same pair of tracks!

It’s not so much of a problem right now as we really only have one train to terminate, the Overlander, and this runs only once a day. But thinking forward a few decades and we could have a lot more regional and intercity trains to accommodate. With any luck we will have a Waikato service within the next year, and once that proves successful the logical progression is to improve the frequency of service and to expand to new destinations such as Tauranga, Rotorua and perhaps Whangarei. Fast forward twenty years and we could need space for six or eight long distance trains an hour at peak times.

The real problem here is not so much the Britomart throat tunnel, but rather the crossovers that sit inside the tunnel. These crossovers are the sections of movable track that shift trains between the two tracks in the tunnel and the five tracks at the stations platforms. With a line under the city we would have trains whizzing into platform one city bound, trains coming from the city whizzing through platform five and at the same time trains going in both directions on platforms three, four and five… all passing through the same diamond of track work. The crossovers simply couldn’t handle this kind of traffic. The end result is a similar constraint to the flat Quay Park junction, but without the opportunity to grade separate.

One way around this is to build a new terminal station somewhere else, or perhaps rebuild the old Strand station to take these intercity trains. I think this is a far inferior option, we have an amazing, custom designed underground rail terminal right in the CBD. This terminal is right were it should be, and it has the unique ability to accommodate diesel powered trains unlike just about any other underground station… so lets use it!

Luckily, there is a bit of space in the tunnel area to overcome the problems at Britomart. If you look closely out the window when catching a train into Britomart you can notice there are actually a pair of unused stub tunnels just before you get to the platforms that were designed for the long abandoned light rail bypass of the station. They only go a few metres before ending in a wall of dirt but they do provide an amazing opportunity for a bypass. The fortuitous thing here is that these little tunnels were designed to allow a track each side to branch off just  before the crossovers, which is exactly what we need! A little extra track and a modified wall each side would mean electric trains headed to platform 1 and on to the CBD tunnel could branch off before the crossovers, and likewise on the other side trains coming from the CBD tunnel and platform 5 could merge back into the tunnel without passing through the crossovers either. The end result is that those super frequent city tunnel trains could fly in and out of Britomart without having to use the same crossovers that would be doing double duty shuffling terminating trains onto the three middle platforms. (if this all sounds a bit complicated, check out the diagram which I hope will explain things better than I can with words).

6) Design new stations and modify Britomart according to the ‘Spanish Soultion’ platform layout, to move big crowds of people quickly.

If some of the previously mentioned fixes are put into place it should be perfectly possible to run sixty trains an hour through the two track CBD tunnel. However running trains this quickly brings us to another problem: the stations would need to be able to support several hundred people each minute moving on and off the platform!

Obviously the stations will each need a high capacity arrangement of escalators, stairs and lifts to move people, but in my opinion we need to go one step further and implement the ‘Spanish soultion’. This is a special arrangement of station platforms that is was to great effect on the Barcelona and Madrid metros, hence the name. Basically this works by separating passengers getting off a train from those who are boarding it. At a regular station there is only one platform on one side of the train and people enter and exit through the same doors on the same side. The main problem with this is you have one mass of people heading out of the train and across the platform at the same time you have another mass of people trying to push through them the other way to board the train. At busy stations this causes major delays and the train has to stop for a minute or two just to get everyone off and on. Obviously this is no good if we want to run a train every two minutes.

The Spanish solution solves this problem by having platforms on both sides of the train, one for boarding and one for exiting. As a train arrives at the station the doors on the exit-only side open first and people hop off to that side. A few seconds later the doors on the other side open and people from the entry-only platform board the train. The effect of this is everyone is moving in the same direction. As there is no jostling of crowds and no waiting for people to hop off before you can hop on, dwell times at stations can be cut to a third of what they would be with regular platforms.

In fact the efficiency of this design extends to the whole station. The usual arrangement for a two track station is to have one wide island platform in the middle that is used by people boarding trains in either direction. This is fed by escalators that go down only, and because people may wait here for trains this is usually where they have seating, vending machines and the like. This is supplemented by two smaller platforms on the outside of each track which are only used by people exiting. These are serviced by escalators that go up only, and because people only use them to exit there is no need for seating or anything else. Effectively having separate boarding and alighting platforms turns the whole station into a conveyor belt system efficiently funnelling people in one way and out the other.

If the CBD tunnel is to support the proposed two minute headways, then I believe platforms of this design will be simply essential to move people through quick enough. In the case of the new stations at Aotea, K Road and Newton it should be no problem to build them like this from the start. All it requires is a slightly wider hole for the station to be built in.

However once again we need to consider the weakest link, which in this case will be Britomart. As it is there is no way Platforms 1 and 5 could handle the passenger flows in and out of trains coming through every two minutes. It would be a perpetual human logjam on the platforms and at the escalators. If we build the new tunnel stations with super efficient high pedestrian capacity platforms, we will need to modify Britomart accordingly. Luckily once again there is some scope to do this. As I mentioned previously Britomart was designed to allow a light rail track to bypass either side of it down at platform level. While none of this was actually built, it does mean there is some space between the interior station walls and the structural box of concrete that encloses the whole station (if you look closely at the coloured walls either side of the platforms you can see a few metres between the interior and exterior walls). This unused space is exactly what we need to build our exit-only platforms either side of tracks 1 and 5, allowing the existing platform faces to be used for boarding only. This could be done fairly simply and cheaply without any major structural changes. All it would require is the interior walls to be removed, relatively narrow platforms to be built in place, and exit-only escalators added to take people up to the concourse level at either end of the station. So not only is this a way to add another six or eight escalators to Britomart, it also means the existing ones can be used much more efficiently.

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