Shippers complained to the Minnesota Railroad commission (P) about the rates for milk carried on passenger trains. P agreed that some of the rates were too high. P then determined that a lower rate from certain towns would be reasonable and brought a mandamus action against D to obey its order to change its rates. P's authority came from a legislative act that mandated that P's final determination of rates was final and conclusive as to what was reasonable and lawful. D challenged that acts constitutionality.