Walkers are now able to take on the 141-mile Heart of Wales trail that runs through a number of counties after the final stage opened on Thursday.
The predicted rise in visitor numbers is expected to provide a major boost to the local economy in the areas. The final stage opened in Llandrindod Wells and was designed by former tourism development professor Les Lumsden.
He said: “Wales is ideal for this type of tourism because one of its greatest assets is its diversity of countryside in close proximity.
“Combined with heritage railways such as this, Wales has the potential to be like Switzerland in attracting walker visitors.
“Previous research shows these trails are major generators of revenue for places that otherwise wouldn’t get the visitor-spend.”
Starting at the train station in Craven Arms, the trail sets out on a journey south west to Llanelli.
It goes past the Brecon Beacons and crosses woodland, wetlands, mountains and valleys.
The railway line, first laid in the 1860s as the Central Wales Line, has survived the threat of closure, dwindling passenger numbers and even tragedy, when four people died in a bridge collapse.
Walkers can now hop on and off at all 29 stations and the potential value to the local economy has been recognised by local authorities, including Powys County Council.
Powys council deputy leader Aled Davies said: “This project is providing real benefits for the people of Powys and the visitors it will attract to the area.
“With hidden gems around every corner, it won’t fail to impress.”
It has taken three years to complete the final and longest stage through Powys which has suffered from neglected paths and difficult terrain.
Volunteers in Powys raised almost £20,000 and Ramblers Cymru said visitor numbers were already rising.
Alan Austin, chair of Powys Ramblers, said: “The idea of setting up a trail that encourages walkers to use the train rather than their cars is a brilliant one.
“We are delighted that Powys council, which is seriously strapped for resources in these times, has supported the trail with manpower and other resources.
“Already there is a noticeable increase in walkers interested in following sections or the whole route.”