2-bay hoppers are used for the most dense loads (such as sand), while 4-bay hoppers are more suited to lighter loads. This is due to axle load limits, for example dry cement is very heavy in bulk quantities and a 4-bay hopper of sand would be very likely to exceed these limits while also having problems with increased strain upon the central span. By trading off cubic capacity 2-bay hoppers are able to transport these heavy loads.