THE average age of trains on Britain's railways is the oldest in at least 15 years, an investigation has found.

A think-tank claimed passengers are "paying through the nose for decrepit trains" while public transport campaigners said there is a "postcode lottery" for train quality.

Rolling stock is now 21 years old on average, Office of Rail and Road (ORR) statistics show.

Press Association analysis found this figure is higher than at any point in publicly available records.

The news comes as Great Western Railway has announced it will be replacing its 40-year-old high speed diesel-powered fleets later this year.

Hitachi Rail Europe will provide new electric and bi-mode trains, for both the Great Western Main Line and the East Coast Main Line.

On their website, Hitachi Rail Europe say: "The Intercity Express Programme will provide an efficient means for passengers to travel on electrified and non-electrified routes, as well as enhancing the customer experience with more modern trains, better interiors and seating arrangements, and of course, faster journey times.

"All of this will lead to a significant improvement in passenger experience and cost savings to the UK government on the long term operation and maintenance of intercity train services across the UK."

While South West Trains have invested £230 million on its new Siemens built Class 707 Desiro City trains which are due to be in service from next November. This coincides with a major £800 million upgrade to Waterloo Station.

The ORR said older trains can result in less comfortable journeys, poorer reliability and worse performance than modern versions, although it also noted that older rolling stock can be refurbished.

Trains in London and south-east England are typically 19-years-old, while "regional services" are 24-years-old.

Rail fares will increase by an average of 2.3% across Britain from Monday.

The Rail Delivery Group, representing train companies, said it expects the average age of Britain's trains to drop to 16 years by 2019 due to the introduction of more than 4,500 new carriages with a capital cost in excess of £7.5 billion.

However, last month public spending watchdog, the National Audit Office, warned that delays in electrification of the Great Western rail line could mean passengers in the North and West of England may have to wait longer for newer trains as the reallocation of rolling stock is held up.

A Department for Transport spokesman said: "We are delivering the biggest rail modernisation programme for over a century and will be rolling out more than 5,000 carriages over the next four years which offer more seats, wi-fi and air conditioning.

"South West Trains will start testing the first of 150 new carriages in the new year and the first new trains on the Great Western mainline will begin operating next summer, before they are brought in on the East Coast route.

"Through rail franchising, we also expect the rail industry to come up with more proposals to introduce new carriages and improve services."

Work was completed by Network Rail last month on the first section of the electrification project between Heathrow junction and Maidenhead as part of the Crossrail programme.

The remaining section will be turned on next year allowing new electric GWR trains to run between Maidenhead and London Paddington from summer 2017. This will allow the introduction of the Elizabeth line trains and GWR’s electric trains to the Thames Valley.

From summer 2017, GWR will extend its electric train services to Maidenhead, which currently stop at Hayes & Harlington.

When the Elizabeth line opens fully in December 2019, passengers in the Thames Valley will be able to travel on new trains right through central London without having to change at Paddington.  

Matthew White, Crossrail Surface Director said: “This vital work is paving the way for quicker, greener, quieter and more reliable trains for people in the Thames Valley.

"Once the Elizabeth line opens fully, passengers will be able to travel right through the capital without having to change at Paddington, making it quicker and easier to get to a range of destinations across London and the South East."