Mass transit systems generally use schematic diagrams rather than geographically accurate maps. This normally has two purposes. One is to make the lines straighter and the other is to make the stations look more evenly spaced. That, in turn, is all for the sake of readability. For example, check out this geographically accurate but impossible to read map of Boston’s T:
Compare that with the diagram:
This all makes sense as practice, but Zhan Guo has a new paper out looking at London which concludes that apparent distance on these schematics makes a big difference to how people plan their trips. A bigger difference, in fact, that the actual travel times. This is something planners need to think about when designing system maps.