The blue and white train is the passenger train. The train with the orange engine is the freight train; It is much heavier than the passenger train so if your bridge can handle the freight train, it can handle anything.

Whats the freight train? The blue one or the orange one

Bridge 6 is also tricky

You are correct. I feel like an idiot now. Thanks for the help.

Ah! i see the problem. You have joined the upper mount point of the stone bridge to the lower mount point on both sides. This isn't necessary as those points are fixed in space and it wastes wood that you need to finish the bridge. Remove these and you will have enough wood to add the two missing struts on each side. See my dodgy picture for a better explanation.
http://steamproxy.net/sharedfiles/filedetails/?id=1303876283
If this link doesn't work, let me know. I haven't done this before.

So I took a screen shot of my bridge. It's exactly the same except two supports that cause the bridge to fail. However, I I don't have it in the budget anymore (ironically it's the same price). Can you tell me where I'm going wrong. I can't see it.

http://steamproxy.net/sharedfiles/filedetails/?id=1303742002

Really? Bridge 5 is pretty reliable for me.
Check that the bridge is reproduced exactly (is the cost the same?)
What parts are breaking first?

Bridge 5 dosn't work.