This is not a perfect drop-on

This is not a perfect drop-on fit.

1. Physical interference: Besides the obvious 4 shell clips that grab the Athearn loco fuel tank and frame there are a lot of obstructions molded on the Athearn shell that needed to be removed to mate with the Atlas frame. In particular what appear to be reinforcements under the walkways and behind the pilots plus around the cab steps do not clear the metal Atlas frame. I am not equipped for accurate metal grinding so I focused on removing shell plastic instead of frame metal.  Unfortunately my Dremel has gone missing so I used #5 and #11 X-acto blades.  Also the air tanks had to be cut off the Athearn shell since they are attached to the fuel tank on the Atlas.
     
2. Cab to frame mounting: the Athearn shell pilots are notched to slip over the couplers, whereas the Atlas pilots have a rectangular hole which the coupler box slides through and is screwed into the frame to hold the shell on. I added strips of scrap styrene behind the Athearn pilots so they can use the Atlas style coupler mounting; a Details West PL-165 snow plow will hide this in the front (the molded-on pilot extensions had to be removed to accomodate the snow plow).  I removed the molded on angled piece over the couplers hoping to use the Atlas coupler boxes but they still interfered. Instead of hacking up the pilot more I just used some Kadee 262 coupler boxes and will live without the angle piece. 
    
    
    
3. Weights: the 4.2 ounce (total) Atlas cast metal weights are a bit too wide and don’t quite fit in the Athearn shell. They would need some filing to fit. Loco is 10.9 ounces without the weights and pulls 10 weighted cars adequately on my flat layout.  This loco will likely be used in an MU config  so I am not immediately concerned about the lighter weight. 
4. Lighting and cab: Atlas has a cab and crew inside the shell and triangular light pipes with 1 large bulb front and rear. Athearn has 2 very small incandescent bulbs front and rear with wires to the circuit board. I really prefer “totally removable” shells without headlight wires from shell to board so I will fashion light pipes and replace the orangey Atlas LED with sunny whites. 

In process awaiting handrails and headlights:

Not a rivet counter exercise but resulting in an appropriate paint scheme for my layout and a smooth running drivetrain using on hand parts. The Athearn mechanism and Atlas CNW shell are surplus and likely to be put on eBay. 

Bottom line, it’s an enjoyable shell swap project if you have the time & motivation, but probably not worth doing if you don’t have a confluence of mismatched shell/drivetrain inventory.   

Jeff

I think I understand, but just for confirmation, please clarify.

When you say Athearn and Atlas, you mean Athearn Blue Box and Atlas "Yellow Box" (), right?  Just for confirmation?

Part numbers

Hi DRGW.  I can't answer your question but the exact locos used were:

Athearn Roundhouse RND14851
http://www.athearn.com/Products/Default.aspx?ProdID=RND14851
it was originally part of a starter set so had no loco box at all, blue or otherwise!  Not sure on how it compares to the blue box pedigree.  

and

Atlas 10000024
http://archive.atlasrr.com/HOLoco/arc-hotmgp382.htm
it is from the Trainman line, the actual packaging was a black and white box.   Not sure on how it compares to the yellow box pedigree.  

thx

Jeff

Nothing in common

The Atlas "yellow box" GP38 was made by Roco starting in the '70s.  The Atlas Trainman GP38-2 uses Atlas "Silver" drive and frame parts and is made in China.  There is absolutely nothing in common between them.

If you start cutting away everything wrong with the old Roco GP38 you will soon have nothing left but the box....

Scott Chatfield