AC/DC

Not the band.

Originally the diesel engine drove a generator that created DC current and ran DC motors geared to the axles.  Pretty much every engine up to the 1980's operated that way.

Then they substituted an alternator that generated AC for the generator and rectified it back to DC to drive DC motors geared to the axles.  The GP38AC and MP15AC engines were that type of drive.

Finally they had an alternator generate AC which was fed to AC traction motors, SD70MAC, C44AC being examples.

What I would imagine the NS engine is is a DC engine that the NS rebuilt into an AC engine.  They replaced the generator with an alternator, the AC traction motors replaced the DC motors and all the control and electronics circuits were replaced to work with AC power.

still curious

What the NS setup is in this loco.  I'll need to be patient.  (why did it 'need' distinctive paint?)

One anecdote to share:  The Pennsy had an electrified freight line parallel to US1 in northern NJ. As a kid (4-5 decades ago), we once took US1 as an alternate, and had the pleasure of pacing an E33.  But the memory that lingers is (from a distance of +/- 30 yards) the odor exhumed from that loco...  25Hz AC catenary power was converted to DC thru Mercury capacitors... and a cloud of ozone followed it along!

Locos long gone, 25Hz died circa 1978 (now uses commercially supplied 60Hz) but This branch still has catenary towers but no overhead wires.. no longer electric service.

What the NS setup is in this

​It touts their rebuilding process.  I Googled "NS ac to dc" and found several articles and dozens of pictures on the rebuilding program.  They are basically turning C40-9's into C44AC's.

Whenever you have a Norfolk

Whenever you have a Norfolk Southern Engine question, the place to start is Chris Toth's NSDash9.com.

Everything you could want to know is there from SD70ACu (Rebuilt former UP SD90MACs) to AC44C6M(GE Dash C40-9W rebuilt to AC with 6 Traction motors which is what you saw)

http://www.nsdash9.com/rosters/4000.html

Norfolk southern is undergoing a massive engine rebuild program. There shops in Juniata (former Conrail Nee Pennsy) and the Norfolk Shops are turning out rebuilds constantly as is GE and Progress rail (EMD) 

2 things are causing this.

1: NS was the longest hold out on DC locomotives. They finally converted to AC recently. 

2: EPA Tier 4 emissions standards apply to new locomotives, but not to rebuilds. By rebuilding their fleet, NS can prolong service life and not have to buy new unproven locomotives.

And to commemorate these rebuilds, they have used some special paint schemes on the first of the units.

In addition to the AC44C6M upgrades and tha aformentioned SD90MACs-> SD70ACu, NS is also converting all there old SD70s to AC.

They've been horsetrading and buying up locos to convert. 

They've also been aggressively upgrading their medium horsepower fleet. Mostly EMD. Both using EMD/Progress ECO stock processes and rolling their own ECO designs. They're also looked into other alternatives like the Progress PR43C some years ago (a failure by all accounts.)

Also, just to follow up what

Also, just to follow up what others have said, up until the 1990s all Diesel electric locomotives used DC traction motors (what turns the wheels) Originally it was Diesel> DC Generator> DC motor. Starting in the 1972 it moved to Diesel> AC Alternator> DC Rectifier> DC Motor

The in the 1990s, AC motors became viable and proved very useful in slow heavy drag freight. So with an AC motor it's Diesel> AC Alternator> DC Rectifier> AC Inverter> AC motor

So a C40-9W is Diesel> AC Alternator> DC Rectifier> DC motor. An AC44C6M is Diesel> AC Alternator> DC Rectifier> AC Inverter> AC motor

So that should show where the DC to AC name comes from.