Question

Why did a provision of the Interstate Commerce Act require railroads to make their rates public

Answer 1

"so that railroad companies couldn’t unfairly charge different rate". By making sure that things like rates were transparent in the market, the government was making sure that there would be no monopolies. 

Answer 2

The Interstate Commerce Act ( 1877 ) is a United States federal law that was designed to regulate railroad industry.

Prior to this act, in many regions of the USA a single company enjoyed the monopoly of railroad transportation, thus dictating their conditions and prices. Many felt this practice was unfair and discriminatory.

The Interstate Commerce act was designed to prevent unjust discrimination by the railroads. It prohibited the pooling of traffic and profits, short haul or long haul fare discrimination and required that railroads to publicize their rates.

By regulating the railroads, the railroads became subject to competition and fairer prices for transporting people/goods were established. To supervise the enforcement of the law, Interstate Commerce Commission was founded.