trains = [('A', 'B', 1.0, 2.0), ('B', 'C', 2.0, 3.0), ('C', 'B', 3.5, 4.5),
    ('B', 'A', 4.5, 5.5), ('D', 'E', 1.0, 5.5)]
Graph with distance and travelling times: find most km's in 24 hours (with constraints)