To provide a rail vehicle and an underframe structure of a rail vehicle having a high assembling performance and high reliability. In an underframe structure of a rail vehicle having side beams 11 coupled to a vehicle structure 1 and bolster beams 13 coupled to a truck, adapters 15 each being attached to the side beam 11 and each having a plurality of projection portions 16 arranged along a vehicle longitudinal direction are provided, the bolster beam 13 includes keys 18 protruding along the vehicle longitudinal direction, and the key 18 and the projection portions 16 are fastened, using bolts BT extending in the vehicle longitudinal direction, in a state where the key 18 is disposed between the projection portions 16 of the adapter 15.