It's nice job - just wish our (British Isles) manufacturers would make such neat end caps!
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I just wandered through a DIY hardware warehouse and looked for the bits.
With 20/20 hindsight I should have used 90mm stormwater pipe and fittings as the tank car is a bit taller than the boxcar so clearance under bridges etc may become an issue.
Well the tanker looks the business - but maybe you could lower the domes if height is a problem?
Lowering the domes would be easy but then oil tankers had fairly high domes from what I have seen.
I have found a couple of pictures of Australian water tankers with relatively low domes/hatches so maybe the use will change.
Built on a wooden flat car kit that I buy from Sri Lanka
Look very similar to the Hartland (HLW) mini series.
http://www.h-l-w.com/mini-gondolas.html
I have a few of the V tipplers and flat cars.
There is a dealer in Sydney....