Rail electrification will lead to "more capacity" and "better connections", says the Transport Secretary.
Justine Greening MP said electric trains would be quieter, and lead to lower maintenance costs in the long term due to less "wear and tear" on the tracks.
The UK Government announces plans to electrify the line from Cardiff to Swansea, as well as several Valleys lines
There are reports that UK Transport Minister will announce a £1bn project to electrify the railway lines in south Wales.
Theresa Villiers will visit the Welsh Assembly to discuss rail electrification for Wales, but isn't expected to announce further plans.