To make the train itself:
Give each student their own square (or squares) of cardboard and access to paints and brushes or rollers. Provide assistance to decorate the square according to their own designs and choices. Encourage students to discuss their plans with each other and compliment each other on their choices - but never try to change the way someone else is doing their design as it is their own choice which counts.
Join the squares together with pipe cleaners and tape on the backs (you may need to poke a small hole in either side of the squares).
As one of your teamwork activities, encourage the group to make choices about the design of the train engine - add round shapes for wheels and any other shapes you desire.
Display the train on the wall - this is one of those great teamwork activities that can grow over time simply by adding more carriages - you may find it eventually stretches right around the classroom!