The diesel-powered trains are to be replaced by a £1 billion "greener" fleet - possibly including double-decker carriages - to cope with passenger growth.

The replacement trains, which could come into service in time for the London Olympics in 2012, will have a more upmarket feel, with scope for airline-style seat-back entertainment systems and broadband internet access.

Darling has taken charge of ordering the trains because he is not confident the private operating companies can handle the process efficiently.

The Intercity 125 ? named after the train?s top speed ? was introduced between 1976 and 1982 and has survived until now through a series of ad-hoc refurbishments.

The ageing fleet, made up of 68 trains, operates on the Greater Western franchise, serving routes from London Paddington to south Wales and the West Country; on the Midland Mainline route between St Pancras and South Yorkshire; and on the east coast main line between London King?s Cross and Scotland.

The first 125 service left Paddington at 8.05am on October 4, 1976. It arrived in Bristol three minutes early.

A return ticket cost £5 ? a quarter the cost of the cheapest available fare today.

In its early days, the train was nicknamed ?the Screamer? because of the high-pitched noise given out by the Napier turbocharger on its 2250bhp Paxman Valenta engines, originally designed for marine use. Until the introduction of the 125, the fastest diesel train could reach 105mph.

However, the average train speed on the network at the time was actually 60mph.

The 125 sounded the death knell for sleeper trains between London and Cardiff, and shaved an hour off journey times between the capital and Edinburgh.

Last month the Department for Transport slipped out a largely unnoticed specification for the new ?High Speed Train 2? contract in the Official Journal of the European Union. This is usually a precursor to formal tendering.

Darling is expected to announce further details of a replacement fleet at a national rail conference in London on March 15.

?The Intercity train has done a great job over the last 30 years and continues to serve passengers well,? he said last night