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Now that rail's Great Western Mainline is not being electrified beyond Cardiff, new bi-mode trains will be introduced on the route this autumn.

Built by Japanese firm Hitachi at a factory in the north of England, there will eventually be 59 of the so called class 800/801 trains on the Great Western Mainline (GWML) from south Wales to London.

They will be operated by the current franchise holder for the line, Great Western Railway, as part of the Intercity Express Programme (IEP), which by 2020 will also see bi-mode trains pressed into service on the East Coast Mainline (ECML) and TransPennine Express routes.

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What is a bi-mode train?

A bi-mode train is a hybrid, using both electric and diesel power.

Where the line is electrified the train will run using electricity from the overhead lines.

And when an electrified section finishes they switch seamlessly to diesel power using its engines.

So, as electrification is only go as far west as Cardiff, they will switch to diesel mode for the journeys to Swansea and vice versa.

But as electrification is not expected to reach Cardiff until late next year or early 2019, then from this autumn they will spend more time in diesel mode on the route from London to south Wales.

What impact will the new trains have on journey times?

Once electrification of the GWML is completed, passengers will benefit from journey times some 15 minutes less than currently from south Wales to London.

Will bi-mode trains be worse for the environment?

They will not be as operationally or environmentally efficient than if new all electric rolling stock had been commissioned on a fully electrified line to Swansea.

But it is fair to say they will be far better on both counts compared to the ageing diesel high speed trains (HST) from Paddington to South Wales.

They will also have the latest Rolls-Royce MTU engines fitted, that are better for the environment.

Lighter than HST trains they can spread their power more evenly across the track, so reducing repair costs.

The wear per mile caused by an IEP train is 56% less than a HST.

Passenger benefits

(Image: ImagesUnite.com 2015)

The IEP fleet will offer around 20% more seats for passengers compared to the existing fleet.

So that up to 652 seats for every full length train - which is 159 more than HST trains.

Seating will be maximised by having longer carriages; some 26 metres compared to 23 metres for HSTs.

They will also have more leg room an personal space for laptops.

The overhead racks will be deeper and transparent, making it easier for passengers to see their possessions.

There will also be extra storage capacity at floor level, with more stacks at carriage ends.

Design boost

Modern engineering means the IEP trains require less maintenance and offer a more comfortable journey experience.

As they are made from aluminium it makes them light and fast - based on the Japanese engineering used to build the world famous bullet trains.

Toilets

The toilets will be bigger than HSTs with wheelchair access. They will also be fitted with baby changing facilities

Wifi and technology

The trains will be fitted with on-board servers that can exchange data using 3G, 4G and high speed wifi connections.

There will also better internal and external display screens that provide passengers with real time information about their journeys.

Modern seat reservation screens will also make finding a seat simpler for passengers.

Screens above every seat will light up either green or red depending on availability and update itself during the journey

Cost

The train operating companies will cover the cost through passenger fares.

However, the whole IEP programme costs £5.7bn.

That includes 122 trains for both the GWML and the ECML, 27.5 years of maintenance and construction of new depots along the length of the both lines.