Yes You are right . Those were days when GOC was not equipped with WDP4B or WDP4D. Even Trichy Madurai was yet to be electrified. So Vaigai was getting Diesel loco from Trichy , KJM WDP 4.
The LSA of that WDP 4 is as follows.
SBC-MAS with Sanghamitra,
MS-MDU...
more... with Pandian,
MDU-TPJ with Vaigai,
TPJ-MDU with Vaigai,
MDU-MS with Pandian,
Even Nellai and Podhigai got KJM WDP 4. Around 8 KJM WDP 4 were regularly running in Chord line that time.
Sanghamitra and 12609/12610 were the loco carriers. Even KJM twins were occasionally sent to Chord line