Such lumber carriers were used on many american saw mills to haul cut lumber to the drying yards or loading docks. This model is desgined to represent the classic Gerlinger style lumber carrier in N scale.
Under the main frame, you'll find two chain covers to be placed on top of the first gear set. See prototype photos for reference.
You can add the stearing rods from 0.4mm wire (holes are allready in place) and the drive chains from brown yarn or fine wire.You need a stearing wheel as well. Maybe a caboose break wheel on wire or something similar.