It looks like a fun project.

I did notice something odd in the example on your github page:

  $ tubemaps path  --from "Euston" --to "Hammersmith"
  # Piccadilly Line: Hammersmith to Barons Court
  # Piccadilly Line: Barons Court to Earl's Court
  # Piccadilly Line: Earl's Court to Gloucester Road
  # Piccadilly Line: Gloucester Road to South Kensington
  # Piccadilly Line: South Kensington to Knightsbridge
  # Piccadilly Line: Knightsbridge to Hyde Park Corner
  # Piccadilly Line: Hyde Park Corner to Green Park
  # Victoria Line: Green Park to Oxford Circus
  # Victoria Line: Oxford Circus to Warren Street
  # Victoria Line: Warren Street to Euston

It may just be me, but it seems you have "from" and "to" reversed?

(NOTE: Trusting my judgement on such directional things tends to be a bad bet since I'm ambidextrous and dyslectic --I don't know left from right, and for me, it often doesn't matter. ;)