They are watertube boilers. Similar to marine boilers. They are the opposite of firetube boilers which were commonly used on steam locomotives where the combustion gasses go through the tubes and water surrounds the tube. In these, the water/steam is inside the tubes, while the fire is outside them. In operation, these are much safer than firetube boilers. And, before you ask, several roads did experiment with watertube (or a hybrid design) boilers... unsucessfully. The B&O was the major road that used a variety of them, but other roads did experiment (including the PRR). The big advantage of firetube boilers in locomotives was the large reservior of water in the boiler that was at the boiling point for the pressure inside the boiler would respond instantaniously to a lowering of pressure inside the boiler due to increased steam demand. Lowering the pressure caused more water to flash to steam without the crew's action. This was valuable in locomotive use, but not so much so in other, steady-state applications.