The promotions apply to several trains running from Monday to Thursday, including services HP1, HP2, LP5, LP6, LP2, LP3, LP7 and LP8.
Under the scheme, groups of four passengers travelling on the same train and route can pay for only three tickets using the discount code 3tang1, while groups of eight can pay for six tickets using the code 6tang2.
Passengers purchasing discounted tickets will not be allowed to change or refund tickets and cannot combine the offer with other promotions.
Vietnam Railways also said it would continue offering unlimited monthly passes for frequent passengers on the Hanoi-Haiphong route.
Monthly tickets cost VND 935,000 (USD 35.5) for shorter routes and VND 1.36 million (USD 51.7) for the full Hanoi-Haiphong journey.
Single fares currently range from VND 104,000 to VND 126,000 between Hanoi and Hai Duong, and from VND 170,000 to VND 198,000 between Hanoi and Haiphong, according to railway officials.
Authorities said the promotions are expected to help reduce travel costs and encourage more passengers to choose rail transport.