This bill, known as the Bus Rolling Stock Modernization Act of 2025, amends section 5323 of title 49, United States Code, to modify regulations concerning advanced payments for bus rolling stock. It allows recipients of federal assistance to make advance payments to transit vehicle manufacturers without needing pre-approval or requiring a performance bond or other security. However, specific conditions must be met for these advance payments. Recipients must have a signed purchase order and an executed contract with the manufacturer that includes advance payment provisions, possess preaward authority, and comply with existing requirements under subsection (m) and section 5318(e) of title 49. Furthermore, the bill imposes a strict limitation, stipulating that an advance payment cannot exceed 20 percent of the total purchase order value for the bus rolling stock vehicle.