I've been to Gettysburg a couple of times, don't remember seeing the train station. I had to Google it to see pics.
Looks like a daunting, but excellent project. Would like to see the results.
Will she be building the 19th century version, or the newley remodeled one?
As far as plans, you could try contacting the Gettysburg Historical Scociety. I'm sure there is probably floor plans for both versions, and for a couple of bucks, copies could be sent to you, which you could scale down to suit her needs.
Or if thats not an option, maybe "guesstimating" the dimensions by using pictures from the net(there's quite a few, from different angles), might be a good method also.
Then there's the ultimate way. Plan a trip to Gettysburg yourself, bring a yard stick and a good camera.
Just a few suggestions.