The designated_direction=* tag describes prevailing direction of trains on a single railway, subway or tram track.
- А multiple track railway usually has a specific direction designated for each track, e.g. "track 1 goes north and track 2 goes south".
- This is not a strict rule as trains may use an "opposite" track for a small part of route under the control of dispatcher, however it is uncommon (example cases are track servicing or emergency situations).
- Specifically, this is not an administrative restriction, so access=* family tags are not applicable, including oneway=*.
- This information is useful for both map users (e.g. knowing the direction I need to go, I want to know which track/platform my train will come to) and mappers (following and distinguishing tracks more easily, and a huge help when adding railway routes).
- The details of railroad infrastructure which set the direction of a track vary from country to country (see http://en.wikipedia.org/wiki/Railroad_directions) and sometimes are neither easy to understand for a common mapper nor suitable for direction deducing by a router/renderer, so a simple generalized "just a direction" tag is needed.
- designated_direction=forward - trains go in the direction of the way
- designated_direction=backward - trains go in the opposite direction to the way
- designated_direction=both - track is used in both directions
Little arrows showing direction of a track, similar to how oneway=* is rendered, however a bit more tuned for railroads (a bit wider perhaps).