Lee
In answer to your question.
The Atlas C628 shell is three separate sections and is better detailed. The Weaver Shell is one piece. MTH bought the Weaver molds for the C628/630 and other than PS2/3 installed and new frame are the same engines.
First 4 photos Weaver Alco frame with QSI sound that I just bought for a Kitbash project. note the lead weights.
Next is a MTH Imperial SD70Ace. Motors and powered trucks are the same. This had been a donor.
My current inventory of Weaver Alcos C628/630 all were upgraded to PS2/3. and lead weights were removed. Engines are still heavy without the lead weights. (Pic was taken in 2014)
N&W C630 PS2, Motors from a Dead MTH SD9, Electronics from MTH RK PA1
SCL 628 PS3, Motors, Electronics from a MTH NS SD70Ace
SP C630/PS2 Motors, Electronics from a MTH CSX SD70Ace