Gwili Railway is a victim of the harsh west Wales weather, with winter rain lashing carriages and rolling stock.
Without cover, the valuable and historical vehicles are prey to the elements, and wooden carriages are easily damaged.
£300,000 is needed for the Under the Roof project, with a mysterious benefactor coming forward to pledge around half of that.
The catch? The railway and its supporters must provide the rest.
The shed would hold four carriages in each of two "roads" within the building.
At stake is the future of the railway, with vital cash now spent on unnecessary repairs being made available to expand and develop the Bronwydd Arms heritage attraction.