The red brick shelter at the Country Park Halt station near Highley was attacked this morning, with a fire reported at 8.40am.
The fire destroyed a roughly 4m square section of the wooden roof, before firefighters from Cleobury Mortimer and Much Wenlock were able to extinguish it using knapsack equipment.
More on the Severn Valley Railway:
Country Park Halt is a small request stop on the Severn Valley Railway heritage line near Highley and Alveley. There is no road access and the trains only stop there by passenger request.
Public journeys on the popular tourist line resumed on Saturday, after a long layoff due to the coronavirus crisis.
Shrewsbury fire station manager Craig Jackson confirmed that the fire was being treated as deliberately started.
He said that a bonfire had been started inside the building, and that it spread to the timber roof.
"We had difficult access to the site and crews had to walk down with knapsacks of water on their backs to extinguish."
A police investigation has been opened and anyone with information is asked to call West Mercia Police on 101.