WDM7s were built by DLW with 2000hp and restricted to maximum speed of 100-105 kmph. Some of them were given to ED at first. Then they were transferred to ERS. For sometime, both ERS and TNP homed WDM7s. Later all of them were transferred to TNP. Now, in the whole of IR, TNP is the only shed to home WDM7s and SR is having the distinction of the only zone to home WDM7. They are restricted to only shunting and other inferior departmental duties. And yes, if you want to see a WDM7, please do come to Chennai because you can never spot it even in other parts of SR too as they never venture out of Chennai limits. :)