Found it quite by accident.

August 1960 TRAINS, an article by J. Parker Lamb. 1959-60 orders of 152 bilevels to replace all existing commuter cars except for 48 "gallery cars" to be modified. Head-end power from 52 EMD F7s modified to contain a HEP generator. GPs to be reassigned. Vague reference to HEP-equipped E8s but nothing specific about the future.

Improvements were coupled with a 24% fare increase and a new zone-type fare structure and what may have been the first monthly pass.

All in all a very interesting article.

JAC

GPRR Thread at TrainOrders