Commuters from North Herts into the City of London will have their 1970s trains switched for state-of-the-art replacements following a £200m deal to restock the fleet.

Govia Thameslink has made the deal with Rock Rail, a new rolling stock company, with support from SL Capital and Aviva Investors.

Siemens will build the 25 new trains, which will enter service by the end of 2018 and replace Class 313 trains built in 1976/77 – the oldest type of electric trains still being used in mainland Britain.

They will run on Great Northern services from Letchworth, Hitchin and Stevenage into Moorgate.

The new trains, a variant of the Class 700 Desiro City, will have climate-controlled air conditioning, power points and free WiFi on board.

Govia Thameslink’s executive officer Charles Horton said: “This new fleet will give our passengers on Great Northern’s suburban routes a modern, high-quality environment, light years ahead of what we have been able to offer to date with the current 40-year-old stock.

“When we bring the Moorgate trains online we’ll also increase the frequency of services considerably in the off-peak, at weekends and even, to a lesser degree, in the morning and evening peaks.”