The Federal Railroad Administration does not currently set any limits on train lengths – and also doesn't regularly track train lengths or their associated risks. That has allowed freight railroad companies to occasionally operate trains up to 8 kilometres (5 miles) long.

Technically, there is no international law limiting the maximum length of a train, but physical and operational constraints typically cap them. In the United States, freight trains have grown significantly, with the median length now around 5,300 feet (1 mile), though "mega-trains" exceeding 14,000 feet (2.6 miles) are occasionally operated by Class I railroads like Union Pacific. The primary limiting factors are the length of "passing sidings" (the side tracks where one train waits for another to pass) and the strength of the "couplers" that hold the cars together. If a train is too long, the physical tension can snap the metal connectors. In Europe, most freight trains are limited to 750 meters (roughly 0.46 miles) to fit within older infrastructure, though trials for 1,500-meter trains exist. The world record for the longest train ever was set in Australia by BHP Iron Ore in 2001; it consisted of 682 cars, stretched 7.3 kilometers (4.5 miles), and required eight locomotives to move.