Im not aware of an "ad" suffix..are you sure that is correct?
"D9" isnt commonly used..although you see it occasionally..
More often you see it written out as: Dash 9-44W, or Dash 9-44CW.
These are locomotives built by General Electric.
"Dash 9" was a "series" of locomotives, of many different models..
In a general sense (and leaving out a lot) GE has made four different "series" of road diesels:
The "Universal Series" of the 1960's, nicknamed "U-boats"..some models were the U25B and U33C..(and many more)
Then came the Dash-7 series, some models were the C30-7 and B23-7 (and many more)
Then the Dash-8 series, C40-8, etc.
Then the Dash-9 Series, C44-9, etc.
and today we have the most recent series, the "Evolution series"
2. the D9-44cw
more commonly written out as Dash 9-44CW
Dash-9 is the "series"
44 stands for 4,400 horsepower
C stands for 6-axle trucks.
(a locomotive such as the B23-7 has 4-axle trucks..)
B = 4 axles (8 wheels total on the locomotive)
C = 6 axles (12 wheels total on the locomotive)
and W stands for "Wide cab"
Some railroads refer to the locomotive as the Dash 9-44CW, other railroads call it the C44-9W,
same locomotive, two different ways of writing it out..
3. The ES44ac
ES stands for "Evolution Series"..the most modern, and current, series of GE locomotives.
the Evolution series replaced the Dash-9 series.
ES = Evolution Series
44 = 4,400 horsepower
AC = AC traction.
always written with uppercase letters: ES44AC..not ES44ac.
and finally the SD70( and it's variations) like the SD70 m-2
The SD70 is not made by GE, it is made by the 2nd major locomotive builder, EMD, which stands for the "ElectroMotive Division" of General Motors.
(EMD is no longer affiliated with GM, but it was for most of its history)
SD= Special Duty..an EMD designation that goes back to the 1950's..
although today, they aren't really "special duty" anymore..they are "regular, normal duty"..but the SD designation is well entrenched,
and EMD hasnt bothered to change it yet!
The "70" in SD70 doesn't stand for anything specific, it is simply a consecutive model sequence that EMD has used for SD models since the 1950's..
SD7, SD9, SD18, SD35, SD38, SD40, SD45, SD40-2, SD50, SD60, SD70, SD80MAC, SD90MAC (and many other models..)
The SD70 uses the old style "spartan" cab..
the SD70M used a wide cab. ("Modern" cab)
The SD70M-2 is an upgraded version of the SD70M..
The "M" in the SD70M originally stood for "Modern cab"..
more commonly called a "wide cab"..
Many times however, when you see an M in an EMD model name, it stands for "modified"..used for rebuilt locomotives.
So the "M" can be quite nebulous..
All SD models are 6-axles..EMD also had the GP series, with 4-axles.
common locomotives like the GP7, GP40 and GP38-2 are in the EMD GP series.
There is a lot more to it that that!
but thats the basic version, to get you started..