Diesel trains are more expensive and require more maintenance
Plans to electrify the main rail route between London and Swansea have been welcomed by transport and environmental campaigners and politicians.
The South East Wales Transport Alliance (SEWTA) said it would mean a new fleet of trains, reduced journey times and general improvements in the service.
WWF Cymru said it would reduce emissions and Swansea AM Val Lloyd said it would bring benefits to business.
But the Campaign for Better Transport is worried about higher ticket prices.
Richard Hebditch from the group said although electrification would reduce costs over time and that generally speaking it welcomed the announcement, the initial outlay might be recovered by the government from passengers.
"We actually want to be growing people using the railway to try and reduce congestion and also reduce CO2 from transport," he said.
"At the moment the government are not being very clear about what the implications are in terms of ticket prices.
"One of the things that we are looking to see is what the increase is next year.
"At the moment most fares are regulated by the government and next January they are forecast to go up by 1% above the inflation rate.
"We are very worried about increasing costs on top of that as well. We would like to see them reduced to the European level."
The government said the £1bn electrification programme would pay for itself over a 40-year period through lower train leasing, maintenance and operating costs.
The short term cost of Network Rail's financing would be met by the Government, it said, and there would be no impact on fares.
Ms Lloyd said the expenditure would be worth it.
"I look upon this as an investment in business in Swansea as well as it being a sustainable form of transport," she said.
Overhead power lines
The electrification could reduce journey times between Swansea and London by 19 minutes.
In the next eight years, overhead power lines will be installed and many tunnels and bridges will be demolished.
Dave Bynon from SEWTA said it had been recommending the electrification of the route for some time because of the benefits to passengers.
"A new fleet of trains, reduced journey times and hopefully general improvements in the service that is provided along the whole of the Great Western main line," he said.
Anne Meikle from WWF Cyrmu said electrification was a good move in the short term to reduce emissions.
"Really the bigger wins come as the whole electricity production system is de-carbonised. The more we have renewables coming in to that grid then the lower emissions for anything that is running on electricity."
First Minister Rhodri Morgan described the announcement as "great news for Wales".
"Faster links to and from London and mainland Europe will vastly improve our ability to persuade investors to move to Wales or expand further in Wales," he said.
"Electrification of the whole mainline to Swansea is crucial in improving accessibility across the whole south Wales region and in encouraging modal switch from private to public transport.
"The use of electric trains and greater use of the train system by passengers and freight will also help reduce carbon emissions."