Question

which best explains how railroad companies were able to standardize their timetables in 1883?
by combining into a single railway company
by working together voluntarily
by unionizing their labor forces
by complying with federal legislation

Explanation:

In 1883, U.S. railroad companies voluntarily collaborated to create standard time zones and standardize timetables, as there was no federal legislation forcing this action, they did not merge into one company, and unionized labor was not involved in this specific standardization effort.

Answer:

by working together voluntarily