Cassandra Pushers

At Cassandra, Pennsylvania, we see four rebuilt Norfolk Southern SD40E locomotives shoving a mail train uphill from an overpass at the railfan look-out.
This portion of track belongs to the Norfolk Southern Pittsburgh Line, handling coal and priority parcel traffic between Harrisburg and Pittsburgh, Pennsylvania in the Allegheny Mountains.
With the ruling gradients on both sides of the mountain upwards of 2% or more, pairs of 'helper' or 'pusher' locomotives help move long and heavy trains up and down the mountain, either by shoving uphill, or by providing additional braking force downhill.