From OpenStreetMap Wiki
To provide additional information about a highway, railway etc
The service tag is used to provide additional information about a highway, railway or waterway.
- service=emergency_access for firefighters
- service=User Defined
The service tag can be used on railway=* lines.
- service=yard - for a railway track forming part of a marshaling yard. Use ref=<index number of the track> for the track reference is known.
- service=siding - for short lengths of track, running parallel to (and connected to) a through-route. Generally used as a "passing zone" for trains, or as a storage area for rail cars (short or long term). In some cases, may be difficult to distinguish from two parallel main tracks.
- service=spur - for short lengths of track, generally built to give one company or entity, access to the main lines. for example a coal-fired power station, a coal mine, a timber-mill, a cement works, etc. Use operator=<name of the operator that the spur serves> for the company served by the line e.g. Portland Cement, BHP Mining, etc.