Bridges, particularly overpass models, differ from railroads as they are not flat but three-dimensional. Therefore, to create an interlocking sense of layers, color depth, contour lines, and shapes are primarily employed, with methods specifically categorized into 3D modeling and flat modeling. Flat modeling is typically used when bridges are considered accessories, while 3D modeling is generally employed to showcase the bridge itself, clearly demonstrating its size and elevation. The specific methods are as follows:

1. Hierarchical Representation: Display the height, dimensions, and levels of the bridge using contour lines. During painting, the color transitions from light to dark from top to bottom, creating a shadow effect for a more realistic visual appearance.

2. Width Representation: During the construction of bridges, visual errors can be utilized to emphasize the width, rather than strictly following proportions. It is advisable to intentionally enlarge certain widths to create a distinguishable sense of width through visual perception.

3. Perspective Representation: Since we typically view models from above, the greenery strips and guardrails on both sides of the bridge should be made taller to ensure they are not obscured.

4. Fixed Model: The completed model must be securely fixed to the base, allowing for piecemeal gluing. This ensures each section is not excessively long and that the joints are reinforced for stability.