That’s an excellent question, and the answer is more complex than a single number. There is no federally mandated maximum length for trains in the United States. Instead, the practical limit is determined by a combination of factors.
Here’s a breakdown of what determines train length in the USA:
1. Physical and Operational Constraints (The Main Limits)
- Sidings and Passing Tracks: This is often the most critical factor. Freight trains must be able to fit into sidings to allow opposing trains to pass. In many parts of the country, especially single-track mainlines, standard siding lengths historically range from 7,000 to 10,000 feet. Therefore, trains are often kept under 8,500 feet to ensure they can clear switches and fit safely.
- Terrain and Curves: Long trains face immense forces on curves and steep grades (hills). On curves, the couplers between cars can be overstressed. On grades, the pulling power of locomotives and the braking power needed to control the train become limiting factors.
- Horsepower vs. Tonnage: There must be enough locomotive power distributed throughout the train (both at the head and in the middle or rear as “distributed power”) to pull the weight without breaking couplers.
- Air Brakes: The train’s pneumatic brake system must be able to recharge effectively. Excessively long trains can have “brake pipe delay,” making the brakes at the rear of the train respond slower than those at the front, which is a major safety concern.
2. Railroad Policies and Labor Agreements
- Individual railroads (like BNSF