There are at least two more examples of aerial sharing networks in the
world I've heard about, aimed at providing indoor coverage : in the Paris
Subway (France) and in the Chigago subway in the US, the latter still
being under construction IIRC.

I know the Paris network very well. It's based on specific underground
network shared by the three French GSM operators. Each operator is
responsible for its own BTS, but the RF distribution network (including
optical links and repeaters) is operated by the transportation authority,
responsible for the  whole RF design and antenna implementation. Aerials
are made of either omni or panel Kathrein dualband indoor products for
stations, corridors and short tunnels coverage, and Jaybeam dualband
yagis in longer tunnels. No radiating cables are used (due to supposed
money saving issues), the 900/1800 feed being carried through the same
coaxial cables (antennas have internal diplexers).

Two of the three operators, Orange and SFR, operate a dual band network
(900 & 1800 MHz, dual BCCH) on this infrastructure. The third one
(Bouygues) only operates at 1800 MHz for the time being.

The global RF design is, as you can guess, very poor. Insertion losses
are of course an issue, but mainly for Bouygues, whose RF outpout power
is strictly limited as all three network use a 1800 MHz component. The
two others cope better on their 900 MHz layer in term of RXlevel and cell  
overlapping.

These issue are not as critical as the high level of intermodulation
products during heavily loaded hours (when mostry all TRX are "on air"),
probably the worst problem they have to cope with.

Moreover, some antenna location choices have been made against the basic
rules of RF design and planning. It's hard being a transportation company
but still thinking one can easely turn into a RF specialist ;-)...

Let me clarify...

Sharing antennas/coax is apparently done in a rare few systems - none in
the U.S. that I'm aware of (yet).  Sharing radios does not happen
because it cannot happen.  A radio can only be connected and controlled
from one switch.  For a BSC radio to be connected to more than one
switch, the control channel would have to have a data splitter and it
would have to lock out any other control channel that wasn't active.  If
a control channel goes inactive, the BSC goes offline and the switch
generates an alarm for that BSC.  Restoring a control channel in most
systems requires a moment or two for each data link (there are always
two), first one, then the other.  The idea of the control channel going
in and out of service due to multiple switches attempting to control the
same radio is simply ludicrous.  A flapping BSC will not be allowed to
return to service due to hold-over timing, thus preventing voice radios
from being opened to traffic just in time for another flap to occur,
removing the BSC from service again, and clearing all voice channels
again.  Did I get that right Lee?  I'm sorry, I didn't introduce Lee...
Lee is a long time RF systems engineer for Cingular visiting me tonight.
 Lee's a good egg, and I'll (he'll) repeat, BSc radios are never shared
- they are always connected to only one switch, and that one switch
always controls all calls on that BSC radio.  All radios in a BSC are
located in the same cabinet and there's only one pair of control
channels for that cabinet.  No exceptions.

Yes, multiple radios can be connected to a single RF radiator (antenna)
using combiners, circulators and splitters.  Virtually all panel
antennas at the top of Cingular's AMPS/TDMA/CDMA/GSM sites are actually
multiple radiators hiding under a single plastic cover, and each
radiator is connected to it's radio cabinet with a separate pair of coax
cables - a pair being one transmit and one receive.  A single radio
cabinet can contain radios for multiple codecs used in the same
frequency band, which is commonly done in 'overlay' systems.  800Mhz
systems and 1900Mhz systems will have separate cabinets for each, and
each cabinet will be connected to it's own antennas via it's own coax.
This can cause a tower to appear to be a co-locate due to more than one
set of panel antennas, but if both sets of antennas are conected to the
same carrier, it's not a co-locate - it is simply two separate service
bands offered by the same carrier at that cell site.

Now, to the sharing of infrastructure... the term "co-locate" means a
tower is shared with multiple carriers, but each carrier still has their
own antennas on that tower, and each antenna set is connected via coax
to indivisdual cell sites (BSCs), and each BSC is housed in a separate
enclosure at the tower base.  This is often considered 'sharing
infrastucture' since each carrier's radios/antennas are sharing the same
tower - but each carrier's radios are still housed in a separate
enclosures, and equipment in each enclosure (BSC) is only connected to
that carrier's switch.  So apparently, 'sharing infrastructure' in most
cases means only the tower is shared, but each carrier still has all
it's own equipment securely enclosed and not shared with any other carrier.

If a BSC radio is connected to a switch, than that switch is the only
control point for that radio, hence it is not shared.  Earlier in this
thread, it was noted that some systems (radios) are sharing antennas -
i.e. one antenna connected to more than one radio, but each radio is
still connected to only one switch, hence only one control point for
that radio.

Switches are not partioned, but billing records have been split ever
since the first roamer made the first call.  A single BSC, connected to
it's switch, can easily handle calls for home users as well as roamers -
each getting their bills separately - but it's still only one BSC, one
switch, and one carrier maintaining the whole enchilada.

I know this is verbose, but...