Sorry, guys - what's the point, if I can't build a bridge that starts out looking like the Eiffel Tower upside-down then falls across the gap just in time to catapult the train forward so it bounces off a cactus and rolls up into a ball that flies back off the left side of the screen and thus technically completes the level?

You can read about how to use it in the resources tab on the same page.

Is there any freeware bridge-building simulation?

We used to do a bridge-building challenge in Structures at uni - the rules were 1m span, loading at mid point, materials cotton thread and balsawood. Winner decided by effiency - load / mass of bridge.

Most students struggled with their one bridge. Using 12 years of balsa aircraft experience, my first bridge was finished in 10 minutes, so I built 2 more - a 'heavyweight' (it weighed nearly 70g!) and my favourite - the 'cheater' which had a ribbon of carbon fibre laminated into the balsa. Most students bridges held 4, sometimes a massive 5kg of barbel weights, with their +500g monstrosities, my 50g main entry failed at 29kg when some fool prodded it and the beam toppled sideways, causing my cotton tensile thread to turn into knives. Hearing 29kg of iron fall 1m onto a concrete floor at the top of a 5-storey 1960's building makes loud bang, and the whole building shuddered. We ran out of weight for my heavyweight, and I sat on the carbon one (I weighed about 55kg back then) and it flexed a little bit.

If I knew the word 'pwned!' then, I would have used it.

Crikey, that's 2/3 the weight of my wife's car! I also like how the new guys say, "That's not bogged!"