Update: The pictured device is a prototype of Joseph Bazalgette's patent sewer maintenance machine, as demonstrated at the Great Crystal Cyberdrome Exhibition. Its boiler is fired by miasma and it is fitted with a variety of cleaning and pest-control ancillaries. Unfortunately the high manufacturing cost and the temperamental nature of their modified rat brains meant that after the initial batch of fifty had escaped Bazalgette was forced to employ men to maintain London's sewers. Even today, you can put your ear to a manhole cover in our capital city and hear the distant clanking of brass wheels on brick walkways and the squeal of exterminated rodents.
Before you ask, no, it isn't a working model.
The main body is made from a plastic Dalek bubble-bath bottle I bought very cheaply at Woolworth's in the post-Christmas sales. At the time I had no idea what I could use it for, but it looked too cool to pass up. I disassembled it and spray-painted the parts with a can of gold Plastikote paint after masking off the two silver arms on the front. The wheels, cylinders, chimney stack, and 'bumpers' came from a rather tacky brass model of Stephenson's Rocket I bought for £5 at a car boot sale. The brass brush on the end of the gun is the head of a rotary wire brush attachment that came with a mini-drill set. The pressure gauge, dome, whistle, safety valve, water level gauge, and valve are all bits and pieces I had lying around the workshop (I used to be into model engineering). All the brass parts were painstakingly cleaned and polished with Scotchbrite, Autosol, and Brasso. It is held together with a combination of screws, hot melt glue, and cyanoacrylate glue. I left the plastic bottle inside the body because the neck acts as the turret bearing - I haven't opened it so it must still be full of bubble-bath!
Update 2: I've locked comments because I was getting quite a lot of spam on this entry.