Engineers will complete the laying of tracks on the new Airdrie to Bathgate railway line on Friday.
The 15 mile stretch is expected to open by December this year, providing four trains and hour between West Lothian and North Lanarkshire and extending transport opportunities between Edinburgh and Glasgow.
The completed route will be the longest domestic passenger railway with new stations to be built in Britain for a century.
On Friday, a 200-tonne mobile track-laying machine ran through the site of the relocated Drumgelloch station, securing the soon-to-be electrified rail track.
Ron McAulay Network Rail director in Scotland, commented: "The completion of this brand new stretch of railway has historical significance but its real significance will become apparent over the next couple of decades.
"In North Lanarkshire and West Lothian, we believe that this rail link will help to rejuvenate the area by encouraging new investment, while at a national level, this line will help to ease rail capacity between Edinburgh and Glasgow and encourage even more people to shift to train travel.
"The completion of track-laying is a major milestone for us and keeps us firmly on track to begin driver training by mid October and to open the line for passenger use by December 2010."
While Network Rail was expecting to complete the track laying process by Friday evening, bosses said there was still a significant amount of work to be done, installing signalling and overhead cables, completing three new stations and relocating two others along route.
However, Mr McAulay said: "We're confident that Network Rail will deliver on schedule."
Transport minister Stewart Stevenson added: "The Scottish Government has ambitious plans for improving the country's rail network and is delivering the biggest transport construction programme seen in Scotland in a generation.
"The Airdrie - Bathgate Rail Link will improve connections between communities in North Lanarkshire and West Lothian providing increased opportunities for business, education and leisure.
"I'm delighted with the very real progress being made on this exciting project. Once complete this new route will offer travelers a real alternative to the car and enable people to make better use of public transport networks."
The new line includes three new stations at Calderdruix, Blackridge and Armadale, as well as two relocated stations at Drumgelloch and Bathgate. Four trains an hour will run on the track, but will operate as far as Helensburgh and Milngavie in the west and Edinburgh in the east.
The £300m project will create the longest domestic passenger railway with new stations to be built in Britain since the Ashendon to Aynho line was constructed through Oxfordshire in 1910.
In Scotland, it's the longest line to be built since the Ballachulish branch line opened in 1903.
