According to the book "C&O Diesel Locomotive" (1994), the RSD-7 is about 8 1/2 feet longer than the RSD-12 and has one additional square filter towards the top of the long hood. Both the long and short hoods are longer on the RSD-7 when compared to the RSD-12.
The RSD12 is short. RSD7s and RSD15s are noticeable longer- note the nose, hence the name "Alligator". They are indestinguishable. To figure out which is which, you have to remember what is not an RSD7. If one is: SP Cotton Belt/SSW DMIR Santa Fe, 800s Pennsy above 8610 it is an RSD15. Anything else is the extinct RSD7. As to an RSD7's prime mover, only one 16-244 exists- it came from an RSD7 and has been put up for sale many times.
