I tried it briefly:
No new MK 2,3,4 electric trains have been added when Electric Trains is installed, but the regular MK 3 and 4 trains are now electric?
I'm a bit confused
Yes. I thought making Mk 1, 2, 3, 4 for burners & the Mk 1, 2, 3, 4 for electric isn't that useful (nobody's gonna use 8 different locos after all).
So the new compatibility is in place to have:
Loco Mk1 -> Loco Mk2 -> Electric Loco -> E. Loco Mk3 -> E. Loco Mk4
Since the electric loco already was about ~Mk2 with its stats, this felt like a fitting progression