By mentioning a double deck I assume you're in NSW. CityRail does have an
V set trailer set up as a club/lounge car. It's not a difficult task to
hire it and have it put in the consist of a regular service. Sydney to
Newcastle and return would give you about 6 hours. There are two options
for organisation, either make all the arrangements yourself or get
somebody with that sort of experience to do it for you; St James Rail
springs to mind.
Another option would be something like an RMS dining car and a few other
vehicles hauled by a diesel (or even steam). I don't know if there are any
"gutted" cars available for dancing in as there are in Victoria. However,
before Steamrail had the empty Moorabool car they used luggage vans 14CW
and 18CE as dance cars, so an MHO/VHO/AHO/MHN would be a possibility. A
saloon car with flipover seats could be used for the actual ceremony.
NSWRTM and 3801 Limited would be the obvious ones to contact for a quote.
Cheers
David