From OpenStreetMap Wiki
| +/- railway=subway
city passenger rail service
Type of railway track for a city passenger rail service (see Wikipedia:rapid transit).
How to Map
Tags to use in combination
- tunnel=yes for underground sections and...
- tunnel=no for overground sections (note that it is useful to set this to dispel any ambiguity in the case of railway=subway ways)
- colour=* to indicate the colour used for each line.
- Railways - general railways mapping information