Determine the Size and Layout

• 1). Consider the model size of your trains when determining how large to make a trestle. An over-sized trestle will make the train seem smaller than it is, while an undersized trestle will make it seem larger. If you know the scale of your model train, use it to determine the size of your trestle pieces. A full-size trestle is generally made up of 12 foot square timbers.
  
  
• 2). Measure the length of the span of the bridge and the height that it will need to be. Use the length of the span to determine how many bents you will need.
  
  
• 3). Consider how you want to support the track of your model train. A bent will not support the track by itself, so you will need to mount the track on a roadbed or install small pieces of wood across the tops of the bents to support the track.