FourNorth - Quad Track Railway - North Dublin

Ways around what, sorry?

In terms of general layout what seems to be the thinking? I think there are near equal benefits to having the slow lines on either side.

The west side allows zero crossover with Intercity trains from the Loop Line to the Northern Line, while also allowing for a DART+ tunnel to emerge on the west side of the tracks at Clontarf Golf Club with no grade separated crossover required. Also allows an airport DART spur to peel off at Clongriffin without having to cross the fast lines.

The east side however allows for DARTs emerging from Spencer Dock to join straight into the slow lines with no crossover required, as well as allowing for some potential peak hour direct Howth trains (along with just a generally smoother platform-platform connection travelling from Howth into the city in the morning). Slow lines on the east also allows a new Fast Line to Drogheda to peel off from the fast lines on the west at Clongriffin without having to cross the slow lines in a complicated flyover.

I think the whole thing comes down to really what way DU is gonna go, because the general two line ‘X’ shape running pattern of DU means that there doesn’t need to be any conflict-less slow line connectivity between the Loop Line and the Northern Slow Lines as they won’t be used for that in regular service.

If a DART spur to the airport is planned as well as a fast line to Drogheda, then there will need to be a flying junction at Clongriffin either way, so this doesn’t have too much influence on the layout.

I think maybe slow tracks on the east has slightly more benefit overall, and even if a tunnel portal at CGC is chosen, the fast lines on the west can just be slewed into the golf club lands, with the DART tunnel ending on the west side, where the tracks climb out of the tunnel and beneath the fast lines to end up on the west side of the line; see the image below (blue is fast lines, green is slow lines and green dotted represents the line descending in cut and cover into the start of a TBM tunnel, with white representing the platform at a relocated Killester).

Any thoughts?

Like @murphaph says the timing of the project is just unfortunate.

In an ideal world I think DU and 4N would be delivered together at which point you can completely design 4N to best work with the design of the portal. However, 4N is just a higher priority for IÉ and I think even in the absolute best case scenario it will open years ahead of DU.

Because of that I think 4N needs the slow lines to easily access the through platforms at Connolly making the western two tracks the best option for the slow line.

I agree with everything you say about the operational benefits, though. I think phasing aside there is more benefit to it being the eastern two tracks, especially if they can swing the IC tracks west in CGC if that is the site chosen.