More than 7,000 new train carriages are being introduced to Britain’s railways, according to an industry body.
The upgrade will enable an extra 6,400 services to operate each week by 2021, the Rail Delivery Group (RDG) said.
Today’s train fleet is among the oldest since records began in 2001.
A number of new trains were introduced following the end of British Rail in the mid-1990s but the average age has risen during the past decade.
The ORR says older trains can result in worse reliability, less comfortable journeys and poorer performance than modern versions, although it notes that older rolling stock can be refurbished.
Travellers using the Caledonian Sleeper service have to put up with Britain’s oldest trains at 42 years old.
New sleeper trains are being introduced on the route between London and Scotland for the first time in more than 35 years, featuring a range of accommodation including compartments with double beds.
Pacer trains which date back to the 1980s and were constructed using parts from buses are still in use in the north of England.
They are being replaced by 281 new carriages featuring wi-fi, power sockets and digital information screens.
Other rail firms set to benefit from new rolling stock in the coming years include Greater Anglia, South Western Railway, TransPennine Express and Merseyrail.
By 2021 a total of 7,033 carriages are set to have been introduced since 2016.
RDG chief executive Paul Plummer said: “With thousands of new, state-of-the-art carriages coming on track over the next three years, the partnership railway’s long-term plan is transforming journeys up and down the country.
“These new carriages will enable rail companies to work together to deliver on their commitment of at least 6,400 extra services a week, better connecting communities and helping to boost local economies from Aberdeen all the way to Penzance.”
The launch of a new fleet of trains costing £5.7 billion in October 2017 was marred by water pouring out of a faulty air conditioning unit and a 41-minute delay.
The problems hit the first passenger service by the new Hitachi-built Intercity Express trains as it travelled from Bristol to London.
Trains in London and south-east England are typically 18 years old, while regional services are 24 years old.
Stephen Joseph, Chief Executive, Campaign for Better Transport chief executive Stephen Joseph said he hoped the new carriages will reduce overcrowding on busy lines.
He also called for rail passengers to be consulted on the design of seats amid complaints that some new ones are not as comfortable as those they are replaced.
“Passengers are paying ever higher fares and expect better trains for their money,” he added.
Rail Minister Jo Johnson said: “This is an exciting time for rail in our country and this year alone will see continuing work on the transformative Great North Rail Project, an entirely new Thameslink rail service and the first trains running though London on the Crossrail project.”