The 3-1-1 designation is a simple way to remember these numbers: individual liquid containers must be 3.4 ounces or smaller, they must fit into one clear, quart-size bag, and there is only one bag allowed per passenger.

The distance between the inner edges of the heads of the rails is known as the Track Gauge. In 2026, the most prevalent measurement worldwide is Standard Gauge, which is exactly 1,435 mm (4 feet 8.5 inches). This specific width originated in 19th-century Britain and was popularized by George Stephenson for his early locomotives. Beyond the standard, railways use "Broad Gauge" (larger than 1,435 mm) for increased stability and capacity, or "Narrow Gauge" (smaller than 1,435 mm) to navigate difficult terrain like mountains more cheaply. In 2026, maintaining a consistent gauge is essential for international "interoperability," allowing trains to cross borders between countries without the need for time-consuming wheel-changing or passenger transfers.