EMU coaches have some cables that run the entire length of the train. These cables carry the control signals for brakes, train lighting (powering the lights and fans in the coaches) etc.
In the attached picture (Source: click here I have highlighted the cables in the red box.
The present generation of AC/DC EMUs running in Madras have five such cables running throughout the length of the rake. These 5 cables (and the sockets they plug into) are labeled as A,B,C,D and E.
Earlier...
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EMUs had these cables dangling outside the motor car (as in the attached picture), however SR started modifying these coaches to have the cables and the sockets hidden under a flap below the motorman's cabin windows. In the original picture in this post, the sockets are hidden under the panel that has the 14007 number written on it. When this EMU motor coach is coupled with another EMU set (Each EMU is made of 3 coach consists, a motor car and two trailer cars), the panel is lifted and the cables from the other EMU coach are attached there.
This change was done to reduce maintenance and improve safety and general aerodynamics (though impact on aerodynamics is very minimal at these speeds).