hump slug
A kind of slug (locomotive accessory) designed for even slower operation than a yard slug, and able to push a long cut of cars over a hump at very low speed.
hump slug: a low-speed pusher for gravity-fed rail yards
A hump slug is a special type of slug, the unpowered locomotive accessory that couples to the back of a powered locomotive to provide extra traction and weight. Unlike yard slugs used for general switching and spotting, a hump slug is engineered specifically for the repetitive, low-speed work of pushing cuts of cars over the hump in a gravity classification yard. It must operate reliably at speeds often under 5 miles per hour while handling heavy loads on an incline.
The hump itself is a raised section of track in a marshalling yard where cars are uncoupled and allowed to roll down by gravity into sorting sidings. A powered locomotive cannot always manage this task safely alone. The hump slug, pushed in front of the locomotive, acts as a controlled brake and stabilizing force, holding back the cut of cars at precise, slow speeds as they crest the hump. This requires robust hydraulic or pneumatic cushioning systems and a heavy frame to absorb repeated impacts from coupling and decoupling.
Construction and weight
Hump slugs are typically 30 to 50 tons, heavier than yard slugs, with reinforced drawgear and substantial frame rigidity. They have neither engine nor generator of their own; they ride on heavy-duty trucks with traction motors that draw power through a cable or hose from the coupled locomotive. The coupling system must be particularly strong because the slug bears pushing loads in both directions and must transmit braking forces without slack or chatter.
Wear and maintenance are significant. The repeated low-speed pushing and the constant minor impacts in a hump yard cause stress concentrations in the frame and coupling hardware. Slack in the main coupling gear will cause jerking, which risks uncoupling cars prematurely or allowing them to bunch and jam. Brake shoes and wheel assemblies require regular inspection and replacement because the slug does not coast or self-propel; every movement depends on the pushing locomotive.
The term hump slug emphasizes its specific purpose. It is not a general-duty slug but a specialized tool for one job: managing the precise, slow control needed to feed cars onto a hump safely and efficiently. Railroads with high-volume classification yards, especially those handling coal, mineral ore, or automotive traffic, may operate several hump slugs around the clock to maintain throughput and prevent derailments caused by excessive speed or slack action at the hump crown.