A couple of observations.
US is a huge market (330m people) - Georgia alone is 10m, but rail freight tends to cross state boundaries, as well as railroad company boundaries. So a short-haul railroad in Georgia will feed NS and CSX mainline operations across the eastern side.
One single customer (the plastics distribution company) will apparently account for 5,000 carloads - if you work on 50 cars per train (big by short haul standards), that is still 2 fully laden trains weekly, all year round. Plus there’s the Port of Savannah traffic etc.
In SA, each line has basically a single traffic - gypsum or grain, and typically only a single customer - Viterra or the gypsum company. More volatile traffic flows. Highly seasonal in grain. Difficult to justify the spend.
For all the economics that the US offers, they still took subsidy from both state and federal government.