The Williamstown Line is a commuter railway line in the city of Melbourne, Victoria, Australia. Operated by Metro Trains Melbourne, it is the city's third shortest metropolitan railway line at 16.2 kilometres (10.1 mi). The line runs from Flinders Street station in central Melbourne to Williamstown station in the inner west, serving 12 stations via Footscray, Yarraville, and Newport. The line operates for approximately 19 hours a day with 24-hour service available on Friday and Saturday nights. During peak hours, the line operates with headways of up to 20 minutes, ensuring frequent service for commuters. During off-peak hours, the service intervals are adjusted to provide service every 20–30 minutes, accommodating the lower demand. Trains on the Williamstown Line run with two three-car formations of Comeng, Siemens Nexas, and X'Trapolis 100 trainsets.

Metro Trains Melbourne, often known simply as Metro, is the operator and brand name of train services on the electrified metropolitan rail network serving the city of Melbourne, Victoria, Australia. It is the largest urban rail network in Australia, with 17 lines and 221 stations across 405 km (252 mi) of railways, and the second busiest network in Australia, with a patronage of 99.5 million as of 2021–2022.