I got challenged to make this as a custom build, I'm fairly happy with it - a steel-frame square tank Nn3 Climax A from my kits.
The mechanism in these is a 25:1 Solarbotics gearhead driving the modified Rokuhan Z power chassis truck, so it's got disturbingly slow speed.
I was always concerned about the degree of difficulty pulling this off with my custom frame and cross-kitting two purchased chassis, but the more I make the better I'm getting at it.
And, BTW, that's code 40 rail on Mark Graulty's (narrowminded) tie strip.
I'm always astounded how big it looks in a photo when this is a 27' long model.