I was having a conversation with someone not too long ago about different methods of switching railroad cars. This person is not a railroader and most of their understanding of railroads has been consequential to associating with me. This person had been perusing a copy of one of my rule books and had seen some references to different switching maneuvers. As we were talking about switching, the topic of letting cars roll on their own came up, and at one point the conversation was interrupted with the question, "what's the difference between kicking, dropping, and humping?"
Apparently, up to that point, I had not explained the different methods very well. Let me also elaborate here, for anyone who is curious.
There are generally two categories of switching cars, flat switching and gravity switching. In gravity switching, generally grades within the yard or switching area are utilized to facilitate the movement of cars. Flat switching requires the use of a locomotive to move cars. When it comes to yards, there are similarly two categories of yards, hump yards and flat yards. Hump yards are yards designed to maximize the ability to gravity switch cars for their classification. Flat yards may not be truly flat, as very little of the surface of the earth actually is, but they generally require the use of a locomotive for most of the classification, as the topography of the yard may only allow for limited gravity switching.
So, how does that relate to kicking, dropping, and humping? Well, let's talk about the difference between kicking and dropping first. Before we get into this, I should explain that in all these scenarios the air brakes have been bled off on the cars in question so that the brakes do not apply when cars are uncoupled from each other or a locomotive.
Kicking cars requires a locomotive but reduces the number of movements a locomotive must make, reducing the time needed to classify cars. To kick cars, the locomotive pushes them towards their destination within the yard. Once they are up to sufficient speed, they are uncoupled, and the locomotive and any remaining cars are brought to a stop. The uncoupled cars continue rolling under their own inertia until some force brings them to a stop. That stopping force may be imparted by a brakeman riding the uncoupled car and applying a handbrake at the appropriate time. It may also be delivered by allowing the cars to couple into standing cars in the track to which they are kicked. Generally, the standing cars will have a sufficient number of handbrakes applied to prevent them from moving when kicked cars hit them. Typically the kick will occur at a location within the yard that allows the next kick to begin right where the previous one ended, minimizing the moves the locomotive must make for each car classified.
Dropping cars relies on gravity to move the cars toward their destination. Rather than having a locomotive push the cars up to speed, the cars start rolling using only gravity. Usually, a brakeman rides a car and applies the handbrake at the appropriate time to stop the cars, just like when kicking. The major difference is that dropping a car does not use a locomotive. In fact, a car can be dropped from just about anywhere just by releasing the brakes, provided it is sitting on enough of a grade to get it moving.
Humping is a variation on both dropping and kicking. Hump yards are designed and built to utilize gravity to streamline the classification of cars. At one end of the yard there is a hill, whether man-made or natural. Cars, or even entire trains, are pushed to the top of the hump, towards the yard. At the top of the hump, a switchman uncouples cars, and they roll away from the crest of the hump, and away from the other cars, into the yard. This is all done as the train is pushed slowly over the hump. Once they roll down into the yard, their speed is controlled by hydraulic or pneumatic retarders in the track. They are allowed to roll into the yard tracks and couple into equipment already in the yard.
Often hump yards employ other methods of automating the classification of cars. Modern railcars have an RFID tag on them which can be used to electronically identify them. Within the railroad's equipment tracking software is a database which identifies destinations and train assignments for all cars. Cars are typically classified by destination, although there are other considerations as well, such as proper hazardous material placement and open load placement. As cars roll over the hump, they are weighed and identified automatically. The computer then uses that information to line all the switches in front of a car and operate the retarders as the car rolls over them. This ensures that the car gets to the right track at precisely the right speed to safely couple to cars already in the track without causing damage to either the equipment or the load it is carrying. As tracks are filled, a yard crew at the other end pulls the classified cars out and assembles them into trains. The efficiency with which cars are classified in a hump yard is really quite remarkable.
Even in flat yards, there is often at least a minor grade. "Flat" is a term used to describe a yard that does not have a hump, despite the fact that very little of the earth is truly flat. Almost every yard has small hills and experienced yard crews use topographical features to expedite switching of cars whenever possible. For example, if there is a slope in one area of a yard, it might be possible to pull blocks of cars up the slope and then drop them back down into the yard, with trainmen lining switches between each drop, to facilitate a quick and efficient classification of the cars.
