This is a Victorian Railways rail tractor (RT). These odd little engines were used for shunting wagons within station limits, and could manage a maximum load of ~10 loaded wagons. They were basically a farm tractor engine and a cab built onto a wagon underframe. You can read more about them here.
RT33 operated at Korong Vale, and I was lucky enough to ride on the back of one when I was around 8 years old. Unfortunately, it was cold and raining on that day, and the driver was so busy shunting trucks that he forgot about me...I ended up with pneumonia, but I still think it was worth it ;)
In terms of the build, the cab took the longest to get right...lots of SNOT to get the window proportions right, and the doors recessed. The external mirrors were also a challenge. I'll probably do a "RT33" decal for the side. You can check out a few more photos here.