Pete speaks the truth, and it's not only Atlas engines! I recently replaced the DCDR with a CC-M in a Lionel C-420, the extra 1/8" was a major PITA! I had to reroute a bunch of wiring to get it all in, the wiring they ran over the top of the DCDR was a no-go with the taller board!
It' even more fun to put the CC-M into the Lionel TMCC Camelback!