Special DLC cars that you could only get from a special edition game (although one is acquired through different conditions). They're all road cars (apart from 1) with a couple of parts added on and slightly better specs than their normal counterparts. The Chrome Line cars are the Audi R8 5.2, BMW M3 Coupe, Mercedes 300SL, Lamborghini Murcielago LP640, the Shelby Cobra 427, and, through a Facebook application, you can also get an XJ13 Race Car Chrome Line. Which is eligible for the Historic Race Car Cup
I think that covers it for the Chrome Line cars...