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There are a couple good explanations of what a classification yard ***is*** but so far no one has talked about ***why*** a classification yard is.
Prior to the prevelance of [**shipping containers**](https://en.wikipedia.org/wiki/Shipping_container) most cargo was carried in box cars and had to either be unloaded / loaded at transfer stations, or the incoming trains broken down and rebuilt into new trains. This was done in classification yards. It’s still done today, but is far less required due to containers being able to be swapped around rather than the entire car or cargo.
Ships bring cargo across the Pacific Ocean from all over Asia. They end up in all the US Pacific ports where they’re unloaded onto docks, and then loaded onto trains. The cargo is destined for all cities across the US, but not trainloads per city per day. So trains leave big ports like Los Angeles and Seattle and head East to cities like Chicago, Kansas City, Fort Worth, etc ([**there are LOTS**](https://en.wikipedia.org/wiki/List_of_rail_yards#United_States_by_states)).
There they are broken down via classification yards, their cars sorted into new trains destined for more specific locations, or possibly even another round of classification sorting at another yard. [**In this video**](https://www.youtube.com/watch?v=hNhmB823-4s) you can see a train being broken down over a hump and the cars rolling into different tracks assembling into new trains. The process is entirely automated today. The car is scanned and identified as it rolls down the hill, and the switches are aligned to sort them into their new destination. There are pneumatic brakes in the rails that slow the cars to the appropriate speed so that they do not smash into the other cars with too much force.
Prior to hump yards engineers had to pull apart trains one car at a time and push them into their new destination rail. Hump yards reduced classification time ***dramatically***.