Just to make it clear, the Auto update is NOT the USA DLC. It is ONLY a patch that updates the game and adds the framework that this and no doubt, future DLC packs can use. The actual USA DLC is over 500mb.
To download the actual USA content, you need to open your steam page, select ‘Train Fever’ and then click on the ‘DLC’ option on the menu on the right hand side, near the top of the screen. You can then download and install it just as you would any other addon, the only difference being there is no checkout as it’s free.