The initial design contract, which was awarded in 2013 was extended for another 20 months in August 2015.
The contract is valued at 60m ($70m).
The extension was due to the expansion of the railway line coverage from an initial 960km to 1,300km.
The Saudi Landbridge project will now extend from Jeddah seaports to Dammam and Jubail on the Gulf coast. The line will pass through the capital Riyadh and other cities such as Mecca and Taif in the Western Province, as well as linking with other transport networks.
Rumaih al-Rumaih, president of Saudi Arabias Public Transport Authority and Saudi Railways Organisation (SRO), said in March that the Saudi Landbridge project will be procured using a public-private partnership (PPP) model.
It was the same original development model envisaged in 2005, when the project was first launched.
The line was proposed to be developed on a 50-year build, operate, transfer (BOT) scheme financed through debt. Four consortiums submitted bids for the concession. After a second round of bidding, a local group of Acwa Power and Saudi Binladin Group emerged as the low bidder, but banks were understood to be wary of providing the quantities of debt required for the scheme and the project stalled.
The PPP plan was consequently dropped and the scheme was relaunched as a state-funded project, with the PIF providing the financing, in 2011.
The project is estimated to cost between $7bn and $10bn.