[Tagging] Relation for saying "x is attached to y"?

[Steve Bennett]

On Fri, Sep 3, 2010 at 9:59 AM, John F. Eldredge <john at jfeldredge.com> wrote:
> The problem is that, even if you have a tag on the shared way indicating "use this only for zoom X or below", and tags on the separate ways saying "use this only for zoom Y and above", there are likely to be some rendering programs that will show all three ways, leading to confusion.

Let's not assume all implementations of any new idea will be faulty :)
Similarly, if the possibility that an existing renderer will badly
render some new tag is considered a blocker, then OSM is essentially
already dead. We're not at the stage where backward compatibility is
essential, IMHO.

That said, I can think of ways to implement such an idea while
maintaining backward compatibility. Let's say three are three ways:

A: north bound train track
B: south bound train track
C: placed between A and B, to represent the higher level abstraction
of the train line, as opposed to the individual tracks.

A: railway=rail
B: railway=rail
C: railway=train_line (or some new, unused tag)

relation Z:
type=grouping
grouping=train_line

Any existing renderer would not render C at all. Any renderer (or
other tool) that added support for railway=train_line, would
presumably also add support for the relation.

Remember, this is all just a hypothetical example, not just about
train tracks. I imagine there are other examples, though I don't have
any in mind atm.

Steve