We haven't really done much in terms of modifications to most of our mainline locos. Most of the changes we've made are just in the physical dimensions, not the loco design itself.

The WDM2 and all it's derivatives (except the WDP3A) use the exact same design as what we got from ALCO (the American company that supplied the initial batch of ALCOs).
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The WAM4, WAG5, flat-faced WAG7, all the WCAM, WCAG and WCG locos use the same design as what we got from the initial WAM1 locos that were imported from France. The only change was in the size of the loco.

The WAP5, WAG9 and WAP7 were designed by Alstom and we use the same design even now. The only modification is that we removed the corrugated side panels that were there in the first batch of imported WAP5 locos (which looked better in my opinion) and replaced them with normal panels.

All the WDP4x and WDG4x locos have the same design as the ones that EMD gave us. The only modification we did were to make the cabs slightly wider and with less sharp edges.

I must correct my original statement. Post-independence, the WAP1, WAP3, WAP4 and WDP3A were the only locos that IR designed completely in house with no external influence. For all others, we are just using the design that were given to use by foreign companies that built them. Creativity in IR loco design ended in the Steam and DC locomotive (WCM series) era.

It is not about which locomotive is best. IR's 3-phase diesel-electric locos (EMDs and the GE WDG4G/6G) and the 3-phase elocos (WAP5/WAG9) are equally efficient. Neither of them is superior to the other. They are made for different purposes.
Countries like the USA and Australia are almost exclusively diesel hauled. They are very efficient and convey a lot more freight traffic than India.
An electric loco can easily be made more powerful, but is useless without OHE. A diesel-electric loco can go anywhere on the tracks, without requiring additional infrastructure.
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reason for going for electrification is the cost and traffic. When a route has very few trains, it doesn't make sense to spend crores of rupees on electrifying the track. But, when a route has a lot of traffic, it is cheaper to electrify it and run elocos on the route.
In recent years, the traffic on IR has increased a lot. So, on most routes, there will be good return on investment for electrification. That's why they are going for electrification of most routes now.
That said, 100% electrification is really stupid. There are still many routes that do not have high enough traffic to justify electrification. Sometimes lobbying and political egos trump common sense. The 100% electrification aim is also on the same line.

The difference is just in the class. The WDP4 and the WDG4 series are practically identical. Only the control software and the gear ratios/weights etc are different between the two classes. So, ideally there should not be any problem in MU'ing them.
But, in this case, I don't think the WDP4 is live. It is connected, yes, but not providing any traction.
It's the same with the WAG5-WAG7 pair as well. Both are based on the same technology and work on the same principle. So, in theory they can be MU'ed.
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I don't think IR uses odd pairs like these often. Even in such cases, both locos might be manned and run independently by the crew. The crew will communicate with each other to maintain the speed/braking etc.

I will try to explain this as non-technically as possible, so please bear with me (May be two or three posts).

Firstly, there are no real '1-phase' locos in IR. We have DC-DC locos, AC-DC locos and AC-AC locos. The AC-AC locos are what we call '3-phase locos'.

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can refer to Wikipedia or Quora for understanding the differences between DC and AC).

Basically, an electric loco takes electricity from the OHE wires and uses it to power motors connected to the wheels. Now the OHE can be carrying either DC or AC current. Similarly, the motors can run on DC or AC current. So there are many combinations possible.

DC - DC : The OHE carries DC current, while the traction motors also work on DC current. All the DC locos in Bombay like WCP1, WCG1/2, WCM1/2/3/4/5/6 and WCAM1/2/3 locos were of this type.

AC - DC: The OHE carries AC current, while the motors run on DC current. The WAM1/2/3/4, WAG1/2/3/4/5/7 and WAP1/2/3/4/6 locos are of this type. In these locos, the AC current from the OHE is converted into DC current and fed to the motors.

Diesel locos like the WDP1/2/3A, WDM1/2/3A/4 and WDG2/3A are also AC-DC locomotives. In these locos, the diesel engine runs a generator (technically called an alternator) to generate AC current. This is converted to DC and used for the motors.


AC-AC: The OHE carries AC current and the motors also run on AC current. The WAP5/7 and WAG9 locos fall under this category.

Among the diesel locos, the WDP4/4B/4D, WDG4/4D/4G, WDG5 and WDG6 locos are also AC-AC locos. In these locos, the diesel engine runs an AC generator (technically called an alternator). The AC voltage generated by this is run through a control system and runs the motors, which also run on AC current.