Rail construction crews in Auckland are sweating to complete critical pre-electrification track work by next week, in time for special trains to run for the Big Day Out festival.

Transport Minister Steven Joyce will announce a key contract for the region's $1 billion electrification project on Thursday, as well as formally opening Newmarket's $35 million replacement station, ready for trains to start using it the following Monday.

Tracks which have been lifted out of the 340m Parnell rail tunnel and beneath St Marks Rd in Newmarket, to lower the ground beneath them, must be relaid by Thursday to cater for crowds heading to the Big Day Out at Mt Smart stadium the next day.

But regular rail services, which have been replaced by buses on Auckland's western line and on the southern line between Britomart and Otahuhu during a feverish round-the-clock construction spree since Boxing Day, will not resume until Monday, January 18.

KiwiRail and its contractors have drafted in about 500 workers to take best advantage of the traditional summer line closures, as the bulk of a $600 million basic upgrade of Auckland's rail networks enters its final stages and the electrification project heads towards the starting gate.

The Auckland Regional Transport Authority also has 100 contractors putting final touches to Newmarket and 20 more working at Kingsland Station in a $6 million upgrade for 2011 Rugby World Cup crowds.

They have been using the summer rail shutdown to build a pedestrian underpass from Sandringham Rd to the station's northern platform, which will need its tracks back by January 18. Completion for the rest of the project has been extended until July.

KiwiRail has been concentrating this summer on ensuring tracks beneath central Auckland road bridges and through the Parnell tunnel are low enough to provide adequate clearance for overhead electric traction wires.

Major work has been taking place under bridges on Mt Eden Rd, Sandringham Rd and Khyber Pass Rd, and 3km of new track will be added to the 4.5km of lifted tracks which need to be relaid before the trains can start running.

A bridge over the southern line at St Marks Rd has also been demolished to make way for a wider but thinner replacement structure, meaning the road will remain closed to traffic until January 25.

That project is taking place beneath piers which Transport Agency contractors are erecting for the $215 million replacement Newmarket motorway viaduct.

The contract to be announced by Mr Joyce next week will be for the supply and installation of train traction wires and their masts within a $500 million Government funding envelope for electrification infrastructure, from which $90 million was committed last year for track signalling.

A further $500 million of government money has been allocated for electric trains, for which KiwiRail is finalising specifications before inviting bids from international suppliers.

The electrification project will begin in stages between now and 2013, starting on the sections of railway between Otahuhu and Britomart, and between Newmarket and Morningside.

KiwiRail projects interface manager Todd Moyle said the programme was aimed at giving those early electrification phases a clear run.

The southern line beyond Otahuhu has been kept open through the Christmas break, and the eastern line through Glen Innes was reopened on Monday after extra tracks and rail "turnout" switching points, with protection against electrical interference, were installed at the Westfield junction.

But Mr Moyle said the biggest challenge had probably been the Parnell tunnel, given the confined space in which workers and machines had to lower the floor and dig drainage trenches to improve water flow.

The floor has been lowered by between 20cm at the Parnell end and 35cm at the Newmarket portal, with careful monitoring of a brick lining installed before the tunnel first opened in 1915.

Mr Moyle said the improved drainage should remove the need for train speed restrictions in the tunnel, a source of frustration to commuters nearing the end of their journeys to Britomart.