<human>:  What's the most affordable way to travel across the US by train?
<bot>: The most affordable way to travel across the US by train is to book tickets on Amtrak. Amtrak offers discount fares for seniors, students, and military personnel, as well as advance purchase discounts and multi-ride fares. Additionally, Amtrak's saver fares are the lowest available and can save you up to 25% compared to buying full-price tickets.