TRSE: How To Draw Tutorials:
The Basics of 3D Drawing

    First thing, have you read the "Reading the Patterns in the Pictures" article? If you don't, you would have no idea what I'm talking, so start reading that important article and then come back to here, because you need to understand how to read and use the patterns in order to be able to understand the concepts of 3D drawings.
      Now that you have some understanding of the fact that everything makes up of imaginary patterns like triangles, squares, circles and other polygons. Let's take step further by learning to THINK IN 3D where everything are made of imaginary cones, boxes, cylinders, balls and any other 3D forms. It has been my observations that many people tend to forget to think in 3D considering the fact that we're living in the world of 3D. Learning to think in 3D may be quite tough for anyone but I will my best as I can to show you the basics of 3D drawings.
    If you look ahead as you walk around around in your schools/universities/long buildings,  you can see how the hallways narrow over distance. A box in the foreground always seem to be bigger than a box in the background even they're of the same size! The lines of the box is always parallel to each other, but since we have natural lens in our eyes, our eyes distorted the way we see a box on purpose to give us a perceptive of depth.
    This is a photograph I took of 1225 ("Polar Express") Engine train from 1940s that was passing through my home village. Compare it with the second one with red lines on it. The "Vectors" lines are the lines that meet at the "Focus Point", and they are very important tools to 3D drawings. The Focus Point is the limit of how far you can see. If you stand between the rails, the rails will seem to meet at the distance, but if you keep walking, the rails will stay parallel to each other no matter how far you have walked. Note how the "boxes" making up the train cars seem to shrink over distance, and it's same for the spaces between the vector lines. The "triangle" represents the front of the Engine Train which two vector lines will form lower corners of the triangle. The steam engine itself is cylinder shaped with circles on the both sides, but if we look at the cylinder from an angled point of view, the circle would seem more of an oval. The circle would seem to "change" its shape as we walk back and forward even the actual shape of the circle stay the same.
    This is a sketch of train I drew as it travels at the foothills of the mountains. Notice how the mountains seem to shrink over distance. But since the train is making a slow turn (rails are arced), the vector lines have to curve to "follow" the curving rails.
    So yes, the 3D drawings are very complicated and is not something for novice drawers unless you're confident enough to take on the challenges of 3D drawings. But there are basic ways you can make your simple drawings look bit 3D.