Peter-Hem - What are the benefits of air brakes over vacuum brakes?
For tourist & heritage trains, firstly compatibility with Tasrail if operating on the main-line, in case of breakdown, easing the logistical challenges for recovery. Secondly, compatibility with any subsequently acquired (preserved) air-braked locos or other rolling stockPeter-Hem - Could somebody give me some information about how the two braking systems work behind a diesel or a steam locomotive?
To much detail to go into here. There is plenty of information on line.Peter-Hem - Is it possible to convert Tasmanian Carriages to both air brakes as well as vacuum brakes?
Yes, dual braking is a common concept. Many British Rail carriages were dual braked.Peter-Hem - How difficult/expensive is it to install an air brake system to a single carriage?
For a professional rail engineering enterprise - not at all difficult. For volunteers, a fair bit of work. Cost, again, dependant on whether the job was contracted out to a rail engineering firm or done in house. Componentry is not difficult to come by or expensive, however the labour to put it all together would be.