Change of Traction from DC to AC. Actually, AC Territory starts from Kalyan Outer to Igatpuri and Kalyan Outer to Pune. At Kalyan, there is Neutral Zone. So trains which come from CSTM/LTT lower their DC Panto, negotiate neutral zone as a dead loco and then again enter AC Territory by raising AC Panto. This is possible only in WCAM type locos which are specifically made to cater to such situations. Pure DC locos are extinct now and Pure AC locos cannot enter DC Territory as their configuration is not meant for such.
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another reason is that there is lack of technical expertise to maintain WCAM class locos, hence WCAMs have a limited run usually around Mumbai/Pune region, but are also seen in far-off places like Amravati (CSTM AMI SF Express), Nagpur (CSTM NGP Duronto), Bhusaval (PUNE BSL Express), Manmad (Panchavati, Godavari, RajyaRani) and Itarsi (LTT-ALD Duronto or CSTM-JBP Garib Rath).
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Hope it is clear to you now.