Return a list of (trip, index).

    trip: a Trip object
    index: an offset in trip.GetStopTimes()

def _GetTripIndex(self, schedule=None):
    """Return a list of (trip, index).

    trip: a Trip object
    index: an offset in trip.GetStopTimes()
    """
    trip_index = []
    for trip, sequence in self._GetTripSequence(schedule):
      for index, st in enumerate(trip.GetStopTimes()):
        if st.stop_sequence == sequence:
          trip_index.append((trip, index))
          break
      else:
        raise RuntimeError("stop_sequence %d not found in trip_id %s" %
                           sequence, trip.trip_id)
    return trip_index