What is the importance of the Circle Line (subway system) in The Last Samurai: A Novel?

The Circle Line (a segment of the London Underground) helps to concretize the circuitous anxiety that Sibylla feels towards her son’s genius. Sibylla and Ludo spend many days aimlessly riding the Circle Line because Sibylla cannot afford to heat their home. Other riders often comment on the dense books that Ludo reads, many of which are in foreign languages. Just as the train follows a continuous, annular path, Sibylla herself returns to the same anxious thoughts over and over again. She worries that, by encouraging Ludo’s intellect, she is enacting terrible harm on her son. On the other hand, she feels it would be wrong to dampen Ludo’s curiosity. Like the Circle Line, her thoughts move in a ceaseless, repetitive circle.