The screen layout:

Number of girders remaining

Add girders

Delete girders

Move joints

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Start the test

Anchor joint

Starting your bridge:

	First of all we need to put the track supporting girders in place. To start this off, drag from the top left anchor point to make a horizontal girder reaching about halfway across the gap.
	You should now have a bridge that looks something like this. Please do not press 'test' yet, the passengers would not be amused.
	Next we need to finish the track off. So drag from the white end of your girder to the right hand green anchor point.
	You now have a complete, but rather unstable bridge. This should be enough to support the train over such a narrow gap.
	This is a far better solution. The extra girder forms a triangle on the left side which is completely rigid. This bridge can easily support the weight of the train.
	When you are happy with your bridge press the 'Test' button. If you want to modify your bridge after a test you can press 'Reset'.

Building a better bridge:

	This is not a good bridge. Square sections are not rigid. The joints are quite weak and will act like hinges. When you put any weight on this bridge it will buckle and collapse. No train passenger wants to be carried over a bridge like this.
	This is a much better bridge. The cross sections form triangles which are rigid. For any of the square sections on this bridge to buckle, it would require enough force to crush the diagonal beam.