A trainload of British, possibly Scottish soldiers
The picture is definetly NOT of Aussie troops but most certainly British troops, possibly of one of the regiments that recruit from the Scottish Lowlands. I can tell this by two facts:
1. There is a soldier to your right that is wearing different headgear from the rest. Look carefully, it is a "Tam o Shanter" bonnet that Scottish regiments wore close to the front line, as an alternative to their steel helmets, once they put away their ceremonial "Glengarrie" headress.
2. Judging by the fact that the men are wearing trousers they are probably from either these Lowland Regiments: King's Own Scottish Borderers, the Highland Light Infantry, the Cameronians, the Royal Scots or the Royal Scots Fusillers
Note: Despite having "Highland" in their title, the HLI recruited from Glasgow, a Lowland city in Scotland, and being Lowland troops, they wore "Trews" (Trousers)