I would like to try to breathe some life into this proposal, albeit with a slight modification. The problem is that marshalling yards and other yards are usually contained in larger areas of railway property. It is therefore useful to be able to distinguish between the railway property as such, and the functional unit of a yard within it. At the moment you can either define the railway property as landuse=railway and then draw all the individual tracks in the area (can be tedious, and not absolutely needed, as tracks in a yard all basically have the same function), or use the landuse=railway to define the yards, and leave the area as a whole undefined.
I would however suggest a slight modification to the proposal, so (applying only to areas) key=railway, value=array. This is because marshalling yards are a particular subset of yards and other track arrays, and we should go for the most generic term. A further refinement would be tracks=n, which would mirror the proposal for this tag for railway lines as such.
If I knew how to (sorry, a newbie, I'm afraid), I would include an example of an area of railway property I have just mapped with five such track arrays, 2 of which are yards and the rest are parallel tracks into maintenance buildings. IMHO, this is more clear than drawing the individual tracks. I have used landuse=railway, but now don't have anything by which to define the railway property as a whole. Until my skills improve, interested are invited to search for 'Taastrup, Denmark' ('Tåstrup' is the same place) --JeffH 19:51, 6 December 2010 (UTC)