Editing Cisco IOS (section)

==== Until 12.4 ====
Before Cisco IOS release 15, releases were split into several ''trains'', each containing a different set of features. Trains more or less map onto distinct markets or groups of customers that Cisco targeted.<ref name="train" />

* The '''mainline''' train is intended to be the most stable release the company can offer, and its feature set never expands during its lifetime. Updates are released only to address [[Software bug|bugs]] in the product. The previous technology train becomes the source for the current mainline train — for example, the 12.1T train becomes the basis for the 12.2 mainline. Therefore, to determine the features available in a particular mainline release, look at the previous T train release.<ref name="naming">{{Citation |last= Rullan |first= John |title= Understanding Cisco IOS Naming Conventions |work= Cisco Academy Conference Presentation |year= 2005 |url= https://www.cisco.com/c/dam/en_us/training-events/le21/le34/downloads/689/academy/2005/BRK-101.pdf |access-date= October 10, 2016 |archive-date= February 8, 2016 |archive-url= https://web.archive.org/web/20160208150227/http://www.cisco.com/c/dam/en_us/training-events/le21/le34/downloads/689/academy/2005/BRK-101.pdf |url-status= live }}</ref>{{rp|6}}
* The '''T''' – [[Technology]] train, gets new features and bug fixes throughout its life, and is therefore potentially less stable than the mainline. (In releases prior to Cisco IOS Release 12.0, the '''P''' train served as the Technology train.) Cisco doesn't recommend usage of T train in production environments unless there is urgency to implement a certain T train's new IOS feature.<ref name="naming"/>{{rp|6}}
* The '''S''' – [[Service Provider]] train, runs only on the company's core router products and is heavily customized for Service Provider customers.<ref name="naming"/>{{rp|6}}
* The '''E''' – Enterprise train, is customized for implementation in enterprise environments.<ref name="naming"/>{{rp|6}}
* The '''B''' – [[Broadband Internet access|broadband]] train, supports internet based broadband features.<ref name="naming"/>{{rp|6}}
* The '''X*''' (XA, XB, etc.) – Special Release train, contains one-off releases designed to fix a certain bug or provide a new feature.  These are eventually merged with one of the above trains.<ref name="naming"/>{{rp|15}}

There were other trains from time to time, designed for specific needs — for example, the 12.0AA train contained new code required for Cisco's AS5800 product.