Paddle = Could be an early signal that placed a board ("paddle") above a track where a train was supposed to stop. If a train didn't stop, the board ("paddle") would hit the stack and alert the crew. As trains grew bigger and faster, such signals became downright dangerous and were no longer used.
Paddling could also describe an early locomotive huffing or cruising along, not working hard. Locomotives with no superheaters (the 119 and the JUPITER at the Golden Spike Monument in Utah: David Kloke's LEVIATHAN and the YORK; the WILLIAM MASON at the B&O Museum in Baltimore) use "wet" steam and produce a subdued exhaust. It doesn't have the "bark" of a modern steam locomotive with a superheater (765, 4449, or 261).
Pig and Pig Pen could refer to stock cars and pens for pigs in shipment.
Hog and Hogger were slang terms for a steam locomotive and an engineer. Following that train of thought, Pig could refer to a locomotive; Pig Pen, to an engine terminal.