We all know that the Northern Line is constrained by the fact that it is predominantly double track, whilst DART, Enterprise and Inter-City/Commuter services all utilise the line.
The solution that has been banded about for years is four tracking the existing line, but even then these plans normally only look at the immediate Dublin area, and don't account for the fact that DART services could one day soon, go all the way to Drogheda.
We also know that plans for improving speeds to Belfast are also banded about. But how can this be achieved in the years ahead with a twisting victorian railway line, that in time, will share up to third of its length with regular, frequently stopping, commuter services. Rail users from Dundalk and Drogheda will equally suffer on trips going into Dublin City Centre.
So, I got my money spending crayons out again. What if we scrapped the idea of quad tracking. Instead, we built a new alignment from South of Drogheda at Colp, that went, as far as possible, on a relatively straight alignment to the airport. Then from the airport, we tunnel, single bore, under the existing port tunnel, exiting at a portal in Clontarf Golf Club. Then we ensure the remaining line to Connolly is quad tracked. This would be 40km of new line, and 8km of tunnel.
Further improvements in time could be carried out on the alignment between Drogheda and Dundalk, and straightening the alignment between Newry and Lisburn (via Banbridge). It would also allow capacity for services to Armagh, Derry and Omagh if a rail line is ever reinstated there.
Finally, if the DART interconnector were to be rehashed, we could potentially look at allowing for additional capacity to enable services from Cork, Limerick and Galway to go onwards to the new Dublin Airport station. All the while the coastal route can be left to serve the local populations along its length more frequently than it would be in the current circumstances.