Kerala express, gets fresh pair of loco for its long run from erode. This loco would have undergone full maintenance and will be out from shed for the long run, this is to make sure that loco don't end up in failure during the mega journey. The pattern of Kerala Express loco is somewhat like this.
12625 will get new loco at ED, which will go till NDLS, return along with 12626, till ERS. From ERS, the same loco will go with 12625 to ED. From ED to shed.
Which means, the loco...
more... coming with 12625 is already reached the KM limit for returning to shed
Hope now you understood why it is getting new loco at ED.