Cattails or Bullrushes are an interesting way to add the idea of water to a scale miniature scene for a dolls house or railroad. They normally grow at the water's edge, or in boggy ground so including them in a model scene implies the presence of water.
The instructions that follow use a wide range of materials. Four different materials are shown in the photo. Pick the material which best suits the scale and time period you wish to model. You can create cattails which look like they are just forming in early summer, or scruffy bullrushes with long dead leaves and gaps pulled out by birds for a winter model.
Cattails or bullrushes have different shapes and sizes according to the variety and where they are growing. Adapt the instructions that follow to create the type most often seen in the situation you are modelling.