Minimum radius = 2nd radius

 

With an express running round the 4th radius outer loop, the inner branch can function as a local passenger or industrial route. Small tank engines and trains (maybe even an autocoach) are the order of the day, with an intermediate stop in the lower left corner. With no run-around loop on the top right station, an isolating section or DCC control will enable the first loco to uncouple whilst a replacement moves from the siding - onto the front of the departing train.