These cranes were used by bridge gangs. Rail mounted, and then when lifting they would secure the crane to the rails to prevent toppling. They would only lift to the side, no ability to swing the lifting arm, so when they had to lift something on the other side of the bridge they would have to turn the crane. A small portable turntable was used for this purpose, usually being placed on one end of the bridge on a small bit of track at right angles to the railway. Useful for lifting beams, piles, crossheads etc from ground level to bridge position. Hand cranked, set up so that people could work on both sides simultaneously. They were often placed at one end of a series of timber bridges, sometimes for years between maintenance cycles; I recall seeing one between Yering and Yarra Glen for years. Presumably easier and more convenient to have a few strategically placed than relocate them for every job. Typically towed behind trolleys, although I assume they could be placed by a team of strong men onto a QR wagon.
The cranes relied on a ratchet assembly to secure the load, so were hardly compliant with modern day standards. I doubt they could be used as intended at all these days. Still, in a different era, without mobile cranes, reliant on people's care to prevent accidents, and used by experienced men they were unremarkable enough. Nice to see one preserved.