Excellent question! Of course, Shadow Lines was also the title of a novella by Joseph Conrad, but one of the reasons why Ghosh appropriated it was that the title of this fascinating work relates to a key concern of postcolonialism - that of borders and boundaries and how they are created and sustained but also how they are easily ruptured and shown to be illusory. Shadow Lines is an amazing story that transgresses multiple borders. Postcolonial criticism examines and criticises man-made boundaries and borders as attempts to define a particular group as against another group ("the other"). Postcolonial criticism attempts to rupture these apparently secure boundaries by examining those who live on the margins of these boundaries and also deconstructing (taking apart) the notion of the other. This is particularly true of the "invention" of India the nation, with the Partition of 1947 which drew imaginary lines across India, creating the countries of Pakistan, Bangladesh and India and also causing much death from the resulting riots.
The narrative in Shadow Lines is constantly transgressing boundaries of space and time, thus giving the novel its title, as the lines that divide places and even times are shown to be easily transgressed - "Shadow Lines."
Consider this quote regarding the inherent fragility of boundaries:
[About seeing the border from the air] But if there aren't any trenches or anything, how are people to know? I mean, where's the difference then? And if there's no difference both sides will be the same; it'll be just like it used to be before, when we used to catch a train in Dhaka and get off in Calcutta the next day . . . (151)
Or consider Robi's disgruntled take on borders and nationalism, when he suggests "...why don’t they draw thousands of little lines through the whole subcontinent and give every little place a new name? What would it change? It’s a mirage; the whole thing is a mirage.” It is these shadow lines that the title of this work refers to, thus establishing the temporary and ephemeral nature of various lines in our lives which we treat as so permanent.