Thanks for the useful statistics. This can explain why IR still uses WAM4. The codal life (The normal working life prescribed in the rules for computing asset life and depreciation in IR's Finance) for Electric locos is usually 40 years. But in general if the locomotive maintenance is very good, it's usage can be extended beyond the 40 year guideline. IR's WAM4's are very rugged and they were less prone to failures through out their lifetime ( The failures during recent times are more due to age rather than problems with the design) and their maintenance is very, very simple. That's why most of them are very well maintained. So most of them are capable of running beyond the 40 year limit.
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Most of these WAM4's are around 35 years old and still have a few years of service left. The WAM4's with JHS, GZB, CNB, BRC and LDH are only used for departmental duties now and are not currently used for running passenger trains. TKD's WAM4s are used in MU'ed pairs to run container services ( I have seen them a few times at BZA station). The rest will probably be retired by 2020 or so.
The codal life is decided based on the economical factors like service offered by the locomotive vs cost of maintenance/manufacture etc. Even if new technologies come around, no locomotive will be retired before it's codal life is complete ( due to the economic factors already mentioned). Only in exceptional cases, locos will be put out of service before their prescribed life time is finished ( Like pure DC locos on WR and CR which cannot be used anymore due to change of traction) . This is not just in IR but happens in every transport system around the world, including air transport.
You can refer to the following document for Codal life all of IR's assets including signalling equipment, coaches, wagons, locomotive parts etc. You can see that the E-loco codal life has been revised to 35 years ( page 2). This was done to be pessimistic and as I said, whenever maintenance is good, the life time can exceed the codal life. ( E-locos have better lifetime than diesel and this doc gives 36 years for diesel locos in page 6. So e-locos can be comfortably used for more than 36 years).
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