From OpenStreetMap Wiki
|railway = diamond|
|Crossing rails that do not interconnect|
|Used on these elements|
|Tools for this tag|
|This article is a stub. You can help OpenStreetMap by expanding it.|
Have a look at Tag:railway=railway_crossing, it is much more used.
Crossing rails that do not interconnect. Nodes only.
In most cases they are passive, but if the angle they cross is too steep, they may be active. Seeon Wikipedia.