Key:railway:preferred_direction
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| Description |
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Indicates the preferred direction of train movement on a railway track relative to the way direction, mainly used for multi-track lines. 
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| Group: railways |
| Used on these elements |
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| Requires |
| Status: de facto |
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The general direction trains travel on a piece of track.
In general, railways are different from roads in that traffic in that the vehicles are bound onto their own track outside of special points and also generally operate under signalised operation only (unlike road traffic which is primarily on-sight). For this reason, the individual tracks of multi-track railways aren't strictly one- or bidirectional like roads are (in the oneway=* sense) but instead have a preferred direction. That means, trains usually travel a certain track in one direction only (either regularly or because what signalling allows) but can switch to the other side during emergencies (e.g. because one track is blocked) or for overtaking.
Possible values are forward, backward and both (see also Forward & backward, left & right). Of these, both should only be used on single track lines.
To specify how traffic in the opposite direction is possible (e.g. under special commands or as if in normal operation), use railway:bidirectional=*.
Regional considerations
Germany
According to EBO, Germany's railway laws, only tracks classified as Freie Strecke (open line) have a preferred direction. A Freie Strecke is defined to be a track which is located outside of a railway station (be it station, yard or service_station but not in a halt). Within a station, all tracks are considered to be equal and so by definition have no =*.
It's still common to find =* on a station's main tracks for various reasons, though. However, it may be preferable to omit these on terminal stations since the same train has to back out in the other direction up to the next switch on either side of the tracks and so can't prefer either direction.
Officially, the station begins at the first station signal (often times a main signal before the switch, sometimes replaced by a trapezoid board on the opposite track) or, if there are no signals, at the first station switch. The existence of a Ra 10 is another good indicator since shunting is only permitted within stations, not on open lines.