Wow, that is a big question because there are a lot of design companies that do this but here is one of the more popular ones:
http://www.sandiasoftware.com/I, like Bill, did it the old fashion way, graph paper, compass, pencils and about 75 erasers. My drawing skills aren't great either put then neither are my computer skills. The guys at Trainboard have a lot of experience with the graphic design packages and will tell you the pros & cons of each one. I once found a German based company that gave a free download on their program. I found it searching the web.