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Steve - If you're referring to the small rectangular box, it was for field looping of the dynamic brake in a multiple unit consist of locos with "field loop" dynamic brakes (old F-units, SD7/9). A maximum of 4 units could be consisted for field looping the dynamic brake. There was a switch on the back wall of the cab that had to be set on the lead unit as to the number of units in the consist with field looping. A three-pronged cable had to be connected between each unit in order for the system to work properly. Second generation power had a different control of dynamic braking called, "potential", but still had the rectangular box to be able to MU with the older units, but wasn't required to have the cable connected for a non-field loop locos in the consist. The field loop receptacle of the 1950s is similar to MU air hoses on modern diesel-electric locomotives (SD70MAC and newer). The hoses are there to MU with older units air brake systems; "modern" units MU air brake system via in cab computer so hoses do not need to be coupled and cut-in. Still need the 27-point control cable connected between units for operation, though. Bob
__._,_.___ Posted by: amtrak347@aol.com __,_._,___ |