Slugs are in regular use on the Portland & Western Toledo Hauler. Here is today's train waiting for clearance to cross the UP main at Albany Oregon, to head west (left) through the wye junction and out to Toledo. The power is:
GP39 - GP40 - slug - GP39 - slug - GP40 - GP39 (dash-2 units). The slugs are mated with the GP40's.
Four-axle power is a good choice for the winding branch line which has a fair bit of ribbon rail, grades over 2%, track speed mostly about 25mph max I think. The railroad gets a whole trainload of traffic from the big Georgia-Pacific mill at Toledo. Curiously, the mill has an entire circle of track around the mill to access the various spurs. Take a look on Google satellite imagery:
Georgia-Pacific mill at Toledo Oregon
From the archives, my pic of the mill area is from 1991.
And a more recent street view of the industrial glory: Georgia-Pacific mill street view
