Black Bridge, near Machynlleth, has been flooded repeatedly in bad weather, meaning the line has to be closed until flood water subsides.
The bridge will be raised by 1 metre in a £3.6m project, which will be enough to reduce any impact from high river levels in the future.
Preparation work began in March. Now, while the main project work is carried out, the line between Machynlleth and Shrewsbury will be closed and a replacement bus service will be in operation.
Buses will be replacing trains from 15th May to 28th June, and a revised timetable will be in operation between Machynlleth, Pwllheli and Aberystwyth.
Black Bridge crosses the River Dulas in Powys. As the river is a fish spawning river, there is a limited time period when the work to raise the bridge can be carried out.
Kevin Giles, senior asset engineer for Network Rail Wales and Borders, said: “This is a big project for us at Network Rail and the first time a railway bridge will have been raised away from flood waters in Wales.
“The issues of flooding on this line go back more than three decades and the result is that large sections of the Cambrian line in central Wales end up cut-off and isolated while repair work is carried out.
“We want to ensure our passengers have more reliable journeys and this project highlights our dedication to making the railway more resilient in Wales.”
Alexia Course, Transport for Wales’ Transport Operations Director, said: “The much-needed work will help us to provide a more reliable service during periods of bad weather and will benefit passengers using this route for many years to come.
“Our team will be working hard to support our customers throughout the improvement work. We understand disruption to services is frustrating for our customers, and we would like to thank them in advance for their patience while this work is taking place.”
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