DART underground - options

DU is not intended for through-running of mainline trains. Attempting this would be phenomenally expensive, as the interim station at St Stephen's green would, in effect, become “Dublin Central” and have to be sized accordingly.

The original CIÉ plan for an underground rail link did envision such a central station, underneath its Temple Bar bus depot, but once Temple Bar was sold off, there wasn't another suitable site anymore.

Exactly. The entire point of the Interconnector is to enable through running and thus finally sweat the true and enormous capacity out of the existing radial heavy rail lines, especially the Kildare and (in future) quad tracked northern line. Connecting these two quad tracked routes together to provide metro like frequency with DART level capacity would change how we view the city entirely.

Any station at Merrion Square would be linked underground to Pearse (where there is already provision on the secondary concourse) and to the Metro at St Stephen's Green

Provided the exit layout and capacity is sufficient you could close any entrances on Merrion Square if needed. The protests are always on Kildare Street though

Agree about gauge not being as big an issue as sometimes made out. In Spain, all high-speed rail runs on Standard gauge (1435 mm), and other services run on Iberian Gauge, 1668 mm - even wider than Irish.

The Madrid Cercanías (equivalent of DART) runs on that broad Iberian Gauge (except for one narrow-gauge line on 1000 mm), but most of the Madrid Metro runs on 1445 mm (no, that's not a typo, it really is 10mm wider than standard gauge, and no, I don't know why), while its two tram lines are Standard Gauge.

And despite this use of four different gauges, Madrid has a superb passenger-rail transport system.

Incidentally, in Tokyo many rail services use Metro lines in the city with through running. The Asakusa metro provides track for the Keikyu Main Line, with through services. Tokyo, however, is very much the exception, and this is due to rail being provided by a lot of different companies.

If it doesn’t allow full-length DART trains to stop in its stations, then there is no point in making it compatible with DART. The purpose of DU is to allow a train on the southwestern overground lines to run through to the northbound overground lines. (Connection to the southbound coastal lines is facilitated already by Phoenix Park Tunnel).

If you limit the length of the trains that can traverse the tunnel, you will end up forcing a service change at Connolly and Hueston from “long DART” to “short DART”, in which case you’d be better off just building a Metro line that runs between those stations.

The Heuston DART Underground station could be built as a cut/cover, but even then, mining may be less disruptive. The intermediate station or stations really couldn’t be. The main DU line will have to pass comfortably under MetroLink at St Stephen’s Green, and the Metro station foundations reach down 35 m underground. That means the DU will have to have a track level that's at least 50 m under ground where it crosses Metro.

Typically, you can rise or fall at most 4 metres every 100 m of track (4% gradient), so there's no way you could get a DU station box close enough to the surface within the distances involved to make cut/cover a practical option.

The need to allow headway between trains means that 2x single-length trains are less efficient in terms of track use than 1x double-length train.

It’s also extremely hard to get drivers, and drivers are paid the same regardless of whether there's 1100 or 550 passengers behind their cab, so I don’t think there will ever be a situation where DART is deliberately reduced to smaller trains.

To be honest, if the railway’s loading gauge had allowed it, I imagine IÉ would have ordered double-deck trains for DART+, but our bridge heights, and the Phoenix Park Tunnel, are too low to accommodate these, I believe.

Avoiding mining for Metro made a lot of sense, as every other station was either at surface, or cut and cover. Having one mined station amongst that would have added a disproportionate amount of cost: it’s not just the effort of mining out the station, it’s the extra procurement, setup and teardown costs for employing a different construction method within the project. For DU, I suspect there will be no way around mined stations for whatever is built in the city centre, which is why I’d imagine DU costing in the same ballpark as MetroLink, despite being just four stations on 5 km or so of tunnel. But the effect it would have on the rest of the DART network would be incredible.

DART underground will likely be a key part of the growing rial network and was made to improve the network in many ways, but it's NOT going to be cheap. There is a lot of talk about the specific benifits of a DART tunnel rather than a more dedicated metro. This includes allowing more through services at Heuston and Connolly(also making more room for terminating IC services), more route options for the DART network, and even some wider network ideas such as direct Cork-Dublin-Belfast services. However the reality is despite being a relatively short tunnel when compared to a metro system, being designed for regular heavy rail rolling stock will make it very expensive.

It is difficult to say, but I don't think there is all that much savings to be made with the D+ tunnel, and at the very least a more dedicated metro line, even if it costed more, would probably be far more cost efficient in terms of capacity. We won't know until it's built, but it wouldn't surprise me if the D+ tunnel becomes more expensive than a comparable metro line.

I don't think they should either, I was just listing some benifits given by the AISRR for the tunnel. Internationally I don't believe it is considered strange to have to transfer between IC stations. The examples they gave were Cork-Dublin-Airport and Cork-Dublin-Belfast. The former I don't think makes sense as with the tunnel I think a better use of the corridor would be HH-Airport services. And I question if the use case for a Cork-Dublin-Belfast service, and I feel as though more dedicated services would serve the the lines better.

Ah I should have been clearer, I don't think it would help Connolly by connecting directly to it. But it could increase capacity at SD allowing it to further take pressure away from Connolly, essentially making it better at what its already intended to do. It could allow some Connolly terminating services to become SD-D+ Tunnel services, or it could make room for Connolly terminating services to terminate at SD instead.

could you just put or bridge, with travelator, to allow it be built at surface level? and connect to the main station?

No? The largest benifits of DU are the overall network wide capacity increases, and not just to the DART network but the IC network as well.

For the DART network where I think it is going to shine is along side FourNorth. As part of D+ the northern line will see during peak times 9tph DARTs, 2tph Dundalk Commuters, and the hourly Enterprise. Four-tracking would free space for 12tph DARTs on the slow line, but there wont really be anywhere to send them other than Connolly as a terminating service. South of Connolly there won't be the space. A similar problem will be seen with the completion of four-tracking HH-Heuston, as it will be limited to 11tph DARTs, and I believe that is because if it was brought up to 12tph there might not be anywhere to send the additional service. The D+ tunnel will create breathing room for the network so we wont have one section at half capacity, while everywhere else is pushing maximum frequency.

For the IC network it likely wont benifit Connolly too much, but it could free a good bit of space at Heuston for additional IC services. The current plans have been to have 4tph from HH terminating at Heuston, with DU all 4 of these services could be sent elsewhere, freeing up the extra space for more IC services.

There are other beifits too such as the potential for HH-Airport services if the airport link is ever built, or allowing more route options for the DART network. However I don't think they are nearly as transformative as the capacity increase.

FourNorth & DART+ West will only serve to highlight the urgent need for DU. There is no avoiding building it IMO. Trams don't cut it. Got off the train at 11am this morning in Heuston and the Luas was absolutely rammed full to Connolly.