The plaque says it well. Passchendaele was one of NZ's major battles of WWI. While NZ does not celebrate Armistice Day (11-11) each year, they do celebrate ANZAC (Australian and New Zealand Army Corps) day on April 25th, the date on which the ANZACS landed on the Turkish Gallipoli peninsula as part of an ill-prepared and executed attempt to force access to the Black Sea and gain access to assist Russia. It was the start of the ANZAC alliance that has continued through multiple wars and has some standing in civil relations between the two countries. Many monuments were erected after WWI to the fallen (NZ lost more men in proportion to its population than any other combatant), but some buildings were erected to commemorate specific groups, such as the ones who had been working for NZ rail before they were killed.
The building is gorgeous, and while very few trains still run from there (the most notable is the scenic Taiere Rail Line from Dunedin to Middlemarch, see
https://www.dunedinrailways.co.nz/ ), the building has been beautifully maintained and is used for quite a few other purposes, such as an annual fashion show where the catwalk is constructed over the rail lines.