How did traveling post offices sort mail while trains were moving?

Traveling post offices, also known as railway post offices, sorted mail while in transit by utilizing specially adapted railway cars. Within these cars, postal workers—often referred to as railway mail clerks—manually separated and organized mail as the train moved between destinations.123

This practice was introduced to avoid unnecessary delays in mail departures and to maximize the efficiency of railway transport. By sorting letters in transit, postal administrations could significantly accelerate delivery times, allowing mail to reach destinations much faster than was possible via stagecoach. This system became the dominant form of mail conveyance in many countries, including the United States and Britain, from the mid-19th century well into the 20th century.21