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B&O Slug B&O Slug...
Now here is a little project that you will want to wait to do.  This is a SERIOUS modeling project that will take some time to complete.  The B&O made two slugs out of two old GP9s.  A slug is a neat little piece of machinery that increases the efficiency of a slow moving locomotive.  At slow speeds a diesel locomotive makes more electricity than the traction motors can handle.  The excess power is routed to the slug, where it powers the four traction motors on it.  The Western Maryland started this concept with their two slugs, 138T and 139T.  When they came under B&O control, the B&O thought it was a good idea and made two of their own, also numbered 138T and 139T.  Of all the numbers to choose, I'm not sure why they duplicated the WM numbers, as those two slugs were still being used.  The C&O also liked the idea and made 140T-150T.  The C&O units are a little different than the B&O ones, so this is a project to make 139T, the B&Os last slug.  139T is still out there working today for CSX.  It wears a CSX paint job and is numbered 1063.

The C&O slugs were usually mated to equally rare GP39s.  This was a semi-permanent arrangement, and certain slugs and their "mothers" could be seen together for years.  The B&O slugs usually operated with a homemade SD20-2.  SD20-2s were modified SD35s.  139T was usually seen with 7704 or 7701. 

The slow speed benefits of a slug were cancelled out above 10 mph or so and the slugs were turned off.  Most slugs were used in yard service because of this.  The C&O even put two slugs with a GP39 some times.  As the set sped up the rear most slug shut off above a slow speed, like 10 mph and the second cut off at a slightly higher speed, like 15 mph.

Below is a Dean Heacock pic of 139T.  Notice there is no fuel tank between the trucks.  A slug holds no fuel, since it has no prime mover (engine), it just uses the electricity from the mother unit.