Yes they are not that heavy. A typical treadmill weighs as much as a king-sized bed with matress and boxspring, or an average dresser with drawers. Putting a treadmill on the second floor is not dangerous in terms of weight. You should consider the height of the room, and that constant wear and tear on the floor-boards, floor-joists, or cross-beams may create a "noisy" zone in your floor.