The marginal cost of more trains being low depends on there being infrastructure at all, and some countries and areas are much better built out for this than others. For example, whats the marginal cost of another high speed train from Tokyo to Osaka? Probably low, the high speed tracks are already there and there is probably ample platform capacity at either end. Move east to Califnornia and whats the cost of adding another high speed train from SF to LA where there wasn't one previously? We are looking at north of 100 billion dollars here for this link and years of work. Quite a mountain of money, time, and effort, compared to e.g. scheduling another plane to land even at a tiny airport in an impoverished country.