Nice to see the Peter Pig 15mm trains getting some exposure. I'm loving your railway gun. A lovely paint job on the loco too. I particularly like the bell pull wire detail! You should do a mortar car like on Buster Keaton's 'The General'
I've been shamelessly utilising the highly versatile ACW railway flat cars for my Russian Civil War Trans-Siberian and Trans-Caspian railways. I've added buffer beams and buffers using small domed or flat headed tacks/nails with some success and have a series of modular 'loads' rail and sleeper stacks, improvised defences and various forms of artillery and MGs.
One earlier locomotive I converted was an slab sided armour-plated version of the Wild West train.
I'm currently working on box cars, water casks, workmen's carriages and even early oil tanker wagons scratchbuilding using cut down 00 and even 009 gauge plastic kits with some success. Unfortunately the US carriage looks too ideosynchratic for me. However, there is lots of fantastic modelling scope so I'm looking forward to browsing your blog for ideas.
I've taken inspiration for my early TSR water tower from US railroad examples and my version is built around the cap from a Tamiya minispray and lots of matchsticks. I liked the look of the log counterweight mechanism so much I shamelessly copied them.