The 100 cars per mile of annual traffic is a rule of thumb being advaned by the investment community, so it isn't just to "break even". I have worked on successful shortlines that had far less traffic per mile, so it really depends on what the oosts compared to revenues are. The connecting Class 1 may offer a generous division of revenue (to get the long haul), or the shippers may be willing to pay a surcharge. Costs depend on the weight and condition of rail, bridges and tunnels, terrain and grades, and other maintenane issues. An excessive number of grade crossings or seasonal/sporadic traffic can have a big influence on things. It also depends on whether the owners are in it for a quick return or the long haul. Competent management and employees are a big advantage.