Saluda Grade in North Carolina was the steepest mainline railroad grade in North America at 5.4%. Think Arosa but with a long heavy freight train. It is now abandoned. Below are some great videos about the routes, and a training video for the crews. Getting over the mountain is a very technical process that I think would be very fun in game!
As one who grew up along the other Southern Railway line into Asheville, I'm legally required to say I'd like to see this route make it into the game at some point. ( ͡° ͜ʖ ͡°) But in full seriousness, it'd be nice to go at it with Saluda in TSW.