Complex sentence: Cassatt wanted to end the rebate practice being undertaken by Standard Oil and Carnegie Steel, both larger shippers, which kept the freight rates extremely low: to do so, the two railroads bought stock in smaller and weaker trunk line railroads.
Split into simpler sentences: Cassatt wanted to end the rebate practice being undertaken by Standard Oil and Carnegie Steel--both larger shippers--that kept the freight rates extremely low. To acheive this, the two railroads bought stock in smaller, weaker trunk line railroads.