DART Underground/Interconnector Tunnel, Dublin, Republic of Ireland

An acronym of Dublin Area Rapid Transit, the original DART project was to make better use of the city's heavy rail infrastructure with more frequent and modern local transport. The investment approved in 1979 to electrify the 25-station route around Dublin Bay from Howth in the north, through the city centre south to the resort and commuter town of Bray, represented a reversal of years of rail decline.

Although it was to be another 15 years before Irish Rail's (Iarnród Éireann/IE) DART expanded with the northern section from Howth Junction to Malahide and from Bray to Greystones in the south, the success of Ireland's only intensive urban rail service had been proven beyond doubt.

Concurrent with DART's early years was the rise of the Irish economy, and although the national population is relatively small, it increased to just over four million by 11% between 1996 and 2006. This is forecast to continue, but more significantly the growth has been largely urban, notably in and around Dublin.

The boom years made structural weaknesses more apparent, however, marked by congestion in and around Greater Dublin, with private transport and buses clogging a road layout quite unsuited to heavy use.

THE PROJECT

The Irish Government has shown a willingness to take bold measures through its Railway Procurement Agency (RPA) to make up for earlier neglect, investing heavily to create a modern transport infrastructure. Added prosperity brought more car ownership, thus public transport developments needed to be of a sufficient standard and with the coverage to encourage people to leave the car at home.

Dublin's Luas tram system opened in 2004, followed by the RPA's ambitious programme that included further Luas expansion and commitment to a two-line metro operation scheduled to begin services from 2012. Under the national Transport 21 investment framework for developments between 2006 and 2015, DART is to be expanded into a true system and with more mode interchanges to take on a far bigger role in the capital's public transport.

Along with the Luas Red Line extension due to open in late 2009, DART Underground is a vital part of rail infrastructure to service the needs of the high-profile Docklands redevelopment in eastern Dublin. Fittingly, much of the area was previously owned by the railway for the sidings that once served now defunct areas of the port.

INFRASTRUCTURE

DART Interconnector (also 'DART Underground') is a projected new 5.2km (3.2 mile) tunnel and connecting lines to be operated only by electric DART stock. Built to the Irish 1,600mm gauge and with 1,500V dc overhead electrification, the new twin-track alignment will leave the original line south of Clontarf Road station, entering the tunnel section just east of the temporary IE Commuter Docklands station that will close with DART Underground's opening.

It will pass under the Docklands area by Spencer Dock, beneath the River Liffey before curving westwards beneath Pearse station and St Stephen's Green towards Heuston, the main line terminus for lines into Dublin from the west, with DART services terminating at Hazelhatch, Co. Kildare. As with the tunnel itself, this will require new route electrification.

Contrasting with the original Luas Red and Green routes built as separate operations without physical connection, the Interconnector will facilitate an integrated suburban railway system. With related electrification of suburban lines north and south of the Liffey, the DMU ('railcar')-operated commuter services in greater Dublin will decline relative to DART operations.

The exact route had not been finalised upon announcement of the Interconnector, with the RPA publishing several options, notably whether the Docklands (Spencer Dock) station would be beneath the Liffey to serve both sides of the river, or on the north side to give direct Luas Red Line interchange. Of the projected five Interconnector stations, only High Street/Christchurch will not link with another heavy rail or Luas service.

Of special note is the future importance of St Stephen's Green at the south of the city's main shopping area, destined to be the interchange of Luas, DART Interconnector and Metro North for onward connection to Dublin Airport. Just south of the Liffey, Pearse station will assume a greater role as the principal DART interchange, due to services being split into two.

ROLLING STOCK

DART stock has been delivered in several batches since 1983, a mix of 60 two- and four-car units. Success brought a platform-lengthening programme, allowing for longer formations being used. System expansion will require substantial new orders, as would replacement of some of the earlier DART fleet.

Details have yet to be published, although the specifications will cover the demands of safe in-tunnel operation. In spite of connection to IE's main lines, the Interconnector will use only DART stock.

SIGNALLING AND COMMUNICATION

Except for the Howth Junction-Howth section, all DART trains currently share tracks and signalling with other IE services. The area's central traffic control is at Connolly. Platforms feature real-time indicators and destinations of arriving services, supplemented by on-train information for passengers. Signage features English and Irish (Gaeilge) language versions. Extensive resignalling of the central area will be required for the realigned DART services.

THE FUTURE

In spite of the Interconnector tunnel being just over 5km long, completion of the expanded electrified DART operations, quadrupling of the busiest of the shared tracks and service restructuring are expected to deliver a four-fold increase in passenger journey capacity to 100 million in the greater Dublin area. Work is expected to begin in 2010, with DART Underground services starting in 2015.