Network Rail has commenced work on a £70 million project to increase capacity on the line between Newport and Ebbw Vale.
Funded by Blaenau Gwent Council and supported by a loan from the Welsh Government, the investment includes laying new track to enable the introduction of a new hourly service.
New platforms will also be installed at Llanhilleth and Newbridge, making them fully accessible for the first time.
A £17m signalling upgrade, funded by NR, will also be delivered at the southern end of the line, with control of the new equipment transferred to the Wales Route Operating Centre in Cardiff.
Engineers have begun vegetation clearance alongside 13 miles of the route, while materials are also being deposited at various lineside locations and construction compounds ahead of the work beginning in April.
The first line closure will take place between April 15-18, to enable NR to carry out a major track upgrade in the Llanhilleth area.
The entire project is expected to approximately 18 months.