The dual-system trains are being procured by leasing company Porterbrook at a cost of around £60m, and will be built at Bombardier's Derby plant, with entry into service scheduled for December 2014. Southern says the class 377/6 will be based on the bodyshell of the Class 379 emu now in service with Greater Anglia, but will be technically similar to the four-car class 377/5 units used by First Capital Connect.

Southern is also exploring the acquisition of a further 116 dual-voltage 177km/h emu vehicles, which would be open to other suppliers. This would be dependent on the DfT accepting the business case for the trains and agreeing amendments to Southern's franchise.

The order for additional Electrostars comes amid continuing uncertainty over the contract with Siemens and its consortium partner Cross London Trains for 1200 emus vehicles for the cross-London Thameslink network. Siemens, which was declared the preferred bidder in July 2011, said at the end of October that it hopes to achieve financial close by January. The first trains were originally due to enter service in 2015 and the last in 2018.

Siemens has declined to say whether the delay in concluding the deal will affect the delivery schedule due to a confidentiality agreement with the DfT. However, any delay is likely to have a knock-on effect on the planned transfer of class 319 emus to newly-electrified lines in the northwest of England and the London – Oxford/Newbury route.

There is also considerable doubt over the Thameslink franchise itself, which will merge the networks of Southern, First Capital Connect, and part of Southeastern. According to its consultation document on the new franchise, which was published in May, the DfT expected to issue an invitation to tender this October, and announce a winning bidder in May 2013 with the new franchise starting in September 2013. Southern would be absorbed into the franchise in July 2015.

However, this schedule has been nullified by the government's review of franchising, launched in the wake of the DfT's mishandling of the West Coast franchise competition, and it is unclear when the process will resume.