6.11.2 Updates and TrainsCryptographically signed update files are used to update FreeNAS®. Update files provide flexibility in deciding when toupgrade the system. Boot environments (page 35) make it possible to test an update.FreeNAS® defines software branches, known as trains. There are several trains available for updates, but the web interfaceonly displays trains that can be selected as an upgrade.Update trains are labeled with a numeric version followed by a short description. The current version receives regular bugfixes and new features. Supported older versions of FreeNAS® only receive maintenance updates. Several specific wordsare used to describe the type of train:96FreeNAS® 11.2-U3 User Guide, Release 11.2• STABLE: Bug fixes and new features are available from this train. Upgrades available from a STABLE train are testedand ready to apply to a production environment.• Nightlies: Experimental train used for testing future versions of FreeNAS®.• SDK: Software Developer Kit train. This has additional tools for testing and debugging FreeNAS®.
这里的train是什么意思？查了柯林斯，实在找不到合适的译文，请大佬指点，谷歌统一翻译成：火车