service=spur is used in association with railway=rail to identify a relatively short lengths of track, built to give one company or entity access to the main line, normally for freight traffic only. For example a coal-fired power station, a coal mine, a timber-mill or cement works.
This tag should not be used for services which provide passenger services, even if the line takes the form of a spur.
How to map
Add a service=spur to the way for a railway which is also tagged with the main feature tag railway=*.
Consider also tagging with ref=* and operator=* if known.
In rare cases usage=military or other may apply, in many cases usage=industrial is applicable.
Do not use this tag with lines that carry passenger traffic.