The NS mixing center in Shelbyville has a few dedicated high hood geeps used by a contracting company. NS runs the trains to the mixer. A contract company then moves the racks for loading and unloading using these engines. These geeps as far as I know rarely ever leave the mixer. Here's a few photos of T79 bring one of these captive units back to the yard in Louisville.
(https://www.jreb.org/ns/proxy.php?request=http%3A%2F%2Ffarm8.staticflickr.com%2F7282%2F9266746472_70b329f769.jpg&hash=9f48cb7e6903a26e6c1a59fe3d79e491665428f0) (http://www.flickr.com/photos/texas_eagle/9266746472/)
T79 (http://www.flickr.com/photos/texas_eagle/9266746472/) by vampirolepis (http://www.flickr.com/people/texas_eagle/), on Flickr
(https://www.jreb.org/ns/proxy.php?request=http%3A%2F%2Ffarm8.staticflickr.com%2F7359%2F9266751328_98cb0a6ab6.jpg&hash=d60a5b3dbd70704b185a820bf85e5cad6da1ab1b) (http://www.flickr.com/photos/texas_eagle/9266751328/)
T79 (http://www.flickr.com/photos/texas_eagle/9266751328/) by vampirolepis (http://www.flickr.com/people/texas_eagle/), on Flickr
(https://www.jreb.org/ns/proxy.php?request=http%3A%2F%2Ffarm6.staticflickr.com%2F5339%2F9263986307_5e03c80668.jpg&hash=3a6f3779ed49f8fd04a2cb939580f65e820c644e) (http://www.flickr.com/photos/texas_eagle/9263986307/)
T79 (http://www.flickr.com/photos/texas_eagle/9263986307/) by vampirolepis (http://www.flickr.com/people/texas_eagle/), on Flickr