What does an engine "need" in it?

Some of us have talked about this before.
Thought it would be a good thread.
I have bought used engines that was damaged.
all I reaaly need in them is
1 the 24 volt motors
2 leds head lights ,direction board.
3 some may want smoker.
4 sound unit w/speaker

I have been asked why is all the boards in side the locos for if you don't need them?
In the old days all we had was lights, smoke

In my mind engines are designed for meeting everyones needs the easiest way possible.

Some of us even remove everything and start over with some sytems.

I have a couple of engines that just have a plug in and out at each end and leads to the motors. nothing else.the lead engine controls them or a battery car.

what do you think.??