You'll need:
cardboard boxes (I bought a three pack of 18 x 12 x 12 boxes from Staples)
hot glue gun and glue sticks
packing tape
acrylic paint in blue, red, yellow, and black
scissors and X-acto knife
peel and stick velcro strips
You need two boxes and a third box to use for additional pieces. Put the first box together, using hot glue to close the flaps (you don't want to use packing tape, as the paint won't stick to it later). This will be the main part of the train. The second box needs to be thinner, so trim the body of it down about 4 inches before shutting the flaps. This is the back part of the train. I cut a piece from the third box to glue over the backside, so there would be no seam on the back of the train.
Next, glue the second box onto the back of the first box, so that it sticks out higher than the first train.
The next step is to cut out the train wheels. I used the third box and a salad plate as a template. I also cut a long strip to connect the wheels. I'm not sure what this is called, but you see it on the real Thomas' wheels :).
Trace around the cylinder on the front of the train, and use an X-acto knife to cut out the circle. Shove the cylinder through the circle. You want a tight fit. Then use packing tape to secure the cylinder on the inside of the train. Follow the same steps for the smokestack. I used a papertowel roll cut in half. The only difference from the head is that I secured the smokestack on the inside by cutting the tube lenth-wise several times on the inside of the train and folding those pieces outward. I taped them to the inside of the train.
Now you have your Thomas costume constructed. At this point I used a ruler and sketched out where I wanted the lines to go. Do this with a light hand. Some of my lines and color notes were hard to cover with paint later.
When the paint goes on is when this bunch of cardboard starts looking like Thomas, so stay tuned...
You can find Part 2 of the tutorial here.