GWA is actually an Australian company (Genesee & Wyoming Australia Pty Ltd) and not just a division of a US company. Once Macquarie own the business, they will own the name. If Brookfield-owned GWI want to take the G&W name out of circulation in Australia, they need to propose a deal which will be to the satisfaction of Macquarie-owned GWA.While GWA operates through a locally registered company, as is the case with all subsidiaries of foreign companies, it is controlled by the US company and the contract of sale will dictate if the G&W name can be used. Often the contract will allow the buyer to continue to use the existing brand for a set period of time, maybe 6 months or so, during which time the business has to be rebranded.
An acceptable deal for Macquarie-owned GWA would be something along the lines of voluntarily renaming itself to something else within a year or so, in return for Brookfield-owned GWI agreeing that any further entry to the Australian market would use a name other than G&W.
The name will change at the time of the sale or shortly after.