Building a TrainGIF, Part 3 
The Underframe

Fig. 5.  The fuel tanks on BNSF locomotives are silver, which I represented with various shades of gray. Try to reveal surfaces and form with shading, such as the dark line at the tank's bottom, a slightly lighter shade above and finally the light gray side of the silver tank. Note the white highlight at the upper left end of each air reservoir. I typically place the light source in my GIFs at the upper left for consistency.

No attempt was made to depict the fine details (such as the piping between reservoirs visible in the photo) since they are too small to try to define in an 8.5 inch square block of color. As you work, keep referring to the GIF at its actual size to evaluate the effectiveness of your detailing.

Fig. 6.  I drew the rear truck over the wheels first. Then, since the front truck is symmetrical with the rear, I copied and mirrored it to avoid drawing it all over again. At this point I noticed that the fuel tank was not spaced equally between the trucks and added one pixel in length to the front end. Discrepancies like this often show up as you work since the mathematical calculations are subject to rounding (no such thing as half a pixel!)

Trucks are interesting exercises since there is so much detail to represent in such a small area. Aim for recreating the basic forms first then experiment with varying shadings and blends of color to suggest detail - you may be surprised at the results.

Fig. 7.  Finishing off the underframe details, I added a very dark green stripe under the yellow stripe, followed by a black line below that. I broke the yellow stripe over the center axles to suggest the jacking posts and again between the second and third axle (as seen in the photo). I also filled in the remaining gaps between the trucks and the frame which really solidified the drawing and made it look like the loco had its feet on the ground instead of floating in mid-air.

Both sets of steps have white stripes on their edges. A pair of solid white pixels looked too vivid as this would represent a stripe 8.5 inches wide instead of its actual width of maybe one inch. I used a lightened shade of the dark green I used for the frame and the effect was vastly improved.