Railroad weight limits are controlled by the condition of the road, as EWG probably learned last winter. Too heavy a point load or too heavy a train results in rolled rails or increased wear/decreased availability. Title 49 Part 213 gives pretty specific guidance on what class a track is and what you can do with it. Other than that, it's up to the railroad to safely and efficiently forward freight based on their prior experience and expertise at railroading. Those who have limited prior experience and/or expertise typically have increased opportunities to gain in those areas.