A Lego 'Pannier Tank' steam engine of my own design, and a GWR-style coal truck too. I tried my best to build something that both looked as realistic as possible AND was compatible with regular Lego train systems (hence the low buffers to match the height of other trains' couplings, and tall cab to allow minifigs to stand inside).
Being a huge fan of steam railways this is my favourite thing I have ever built. Oh and that number '1' on the sides means that this is the first of many. Long-term I plan to build an entire Lego railway layout but for now I'm having fun trying to create specific types of engines and rolling stock.