The doc train that allowed people living in the territories to decide the issue of slavery through their governmental bodies was called

The doctrine was known as popular sovereignty

It defines that people of federal territories could decide whether to adhere to the Union as free or slaves, in this sense, the status of slavery could be determined by the settlers themselves.

The doctrine was mainly promoted by Stephen A. Douglas in 1854. His ideas were based on the fact that settlers should decide on their status on early stage of settlement, while other supporters thought of the doctrine as to be applied just before each territory formally establishes or fully prepared for statehood. It had an advantage which was the reduction of conflicts over slavery issues but remained less precise as to what stage of territories development should the decision be made.