The B&O used these units primarily as helpers. Helpers are used to push at the back of a train up a steep grade. The famous Sand Patch grade just west of Cumberland, MD is a perfect example. Usually assigned in pairs, the SD40-2's would wait at the bottom of the hill, hook up to the back of the train needing help and they would head up the hill. At the top, the helpers would discouple and head back down the hill to help the next train. Today some trains are made up with Distributive Power or DP and the two or so engines at the back are run by remote control from the engineer in the lead locomotive.
As engines with six axles the SD40-2's really make their money at slow speeds. At slow speeds the diesel motor creates more electricity then can be used by 4 traction motors. A locomotive with 6 traction motors (1 per axle) can use the extra electricity and pull even harder. At high speeds of about 12 mph or so, this benefit is negligable. That is why the B&O used SD7 and SD9's in yard service where slow speed pulls were the norm. SD40-2's were used in helper service for their great slow speed characteristics.
Below is a Dean Heacock pic of #7619
the last B&O SD40-2. These units were all delivered from the
factory in the Chessie paint and all had the slotted battery box cover.
It was built in Feb 1977 and got repainted into CSX #8261. It still
rides the rails today.
Below is a pic of my #7619. It is a dummy that came from San Diego. I got it for only $10, so I had to buy it. It was originally decorated for the Norfolk and Western, but I stripped off that paint and gave it a nice Chessie paint job.
Below is a starting pic.
Below are a few finished pics.