[Tagging] Public transport zones

[Steve Bennett]

Hi all,
  I don't see any wiki documentation on public transport zones: that
is, how to tag that station X is in zone 3, station Y is in zone 2,
for systems where the price of a ticket depends on the zones
travelled.

For example, in Melbourne, there are two zones. Some train stations
are in zone 1, some in 2, and some in the 1+2 overlap. Travelling from
1+2 to 2 only requires a zone 2 ticket, which is cheaper than a zone 1
ticket. Travelling from 1 to 2 requires a 1&2 ticket, which is the
most expensive.

Any recommended tags? Otherwise, I was thinking of something like:

ptv:zone1=yes
ptv:zone2=yes

or maybe:
ptv:zone=1
ptv:zone=1+2

(PTV is Public Transport Victoria...)

Steve