The steam engine provided on steam locomotives is

Double-Acting: In a double-acting steam engine, steam acts on both sides of the piston alternately during each stroke. This contrasts with single-acting engines where steam acts only on one side of the piston, and the other side is usually open to the atmosphere. Double-acting engines are more efficient because they utilize steam pressure on both sides of the piston, producing power during both the upward and downward strokes.

Non-Condensing: A non-condensing steam engine exhausts steam directly to the atmosphere after it has done its work in the cylinder. This is in contrast to condensing engines, which use a separate condenser to convert steam back into water after it exits the cylinder. Non-condensing engines are simpler and cheaper to build but are less efficient because they don't reuse the steam. They also require a constant supply of fresh water and fuel to operate.