Noob: Can I edit Valve's props?

Hi,

I'm creating a train yard in the level I'm working on, but none of the railway track props are quite the right size, so I've downloaded the XSI mod tool and GCFScrape, and was hoping to edit some of the props to better fit my map, but I haven't a clue how to do it.

Can anyone help??

Thanks

Excellent, thanks for your help.