And keys should always face the direction travel, I.E. they tighten in the same direction of train travel. If it's a single branch line then I think some have to be placed the other way around, could be alternate or six one way or six the other, track experts would know more as I suspect each region or area differed.
It depends on traffic and conditions. If locos are working hard, say accelerating or climbing uphill, then the rail will creep away from the direction of travel, due to reaction.
Conversely, if there are going to be some heavy breaking forces then the rail will creep with the direction of travel. On a single line, keys are likely to be driven downhill on a gradient, and towards a stopping place.
On double track, it will not always be the case that the rail creeps in different directions on the up and down lines: on a steep gradient, it is possible for the keys on both lines to be driven downhill.
That said, local conditions will always prevail, as the gangers will soon find keys working loose if they are driven from the "wrong" side.
A lot of it, though, is down to common sense - but if you can find photographs of track in similar circumstances to those for your model, then use these as a guide.
(By "driven from", this is the side the key sticks out, so it can be hammered in.)
Hope that helps.
). My appologies if this isn't the same news to you as it was to me
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