Walker presents the idea that "routes" can be temporary and impermanent, while "lines" are more immutable:
"Lurking inside these two words, in short, is a profound difference in attitude about a transit service. Do you want to think of transit as something that's always there, that you can count on? If so, call it a line. We never speak of rail routes, always rail lines, and we do that because the rails are always there, suggesting a permanent and reliable thing."