In 1976, Conrail began rebuilding GP7's into GP8's, assigning them to the 5400-5462 series. Some units were initially numbered 5720-5728 but in 1978 were renumbered to 5400-5408 so the GP8's and GP10's could be in solid number blocks, with room for additional units as they were built. Illinois Central Gulf's Paducah shops, Morrison-Knudson's Boise shops, and Rock Island's Silvas shops performed the work. The last GP7 was rebuilt in February 1979, becoming CR 5436.Conrail's GP8's contained a 16-cylinder 567C or 567BC EMD prime mover that produced 1,500 horsepower. The units weighed 249,00lbs. By 1991, Conrail had retired all their GP8's. CR 5437 was equipped with Harmon Select-A-Power, which is noted in a decal on the cab either above or below the 'GP8' locomotive class. Harmon Select-A-Power was an add-on fuel-saving electronic device. When a train didn't need all its units to be online, this device allowed an engineer to take trailing units off line one-by-one until only the lead unit was active. Units could also be reactivated as needed. When a unit was offline, it produced power of throttle position #1. 
Comprehensive EMD GP8 Roster
I created this roster on January 25, 2019. It includes Conrail's final numbering for their GP8's, along with the former Conrail numbering for 5400-5408 before they got placed into the final numbering series. It also includes the GP7's that were rebuilt, along with rebuild dates and the rebuilder as mentioned above. I have no plans of adding dispositions due to their complex nature. Information for this roster comes primarily from Conrail Motive Power Review: 1986-1991 (1992) by Paul K. Withers, Conrail Motive Power Review, Volume 1: The First 10 Years 1976-1986 (1992) by Gordon Lloyd Jr. and Louis A. Marre, various Conrail Locomotive Data rosters, and Conrail enthusiasts around the world.
CR # Former Prior Previous Rbld'r Rebuilt Removed Note
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5400 CR 5720 CR 5836 RDG 634 ICG 09/30/76 04/11/91 1
5401 CR 5721 CR 5620 P&E 5620 ICG 10/20/76 04/11/91 1
5402 CR 5722 CR 5613 P&E 5613 ICG 10/29/76 04/11/91 1
5403 CR 5723 CR 5619 P&E 5619 ICG 11/09/76 04/11/91 1
5404 CR 5724 CR 5947 EL 1217 ICG 11/11/76 04/11/91 1
5405 CR 5725 CR 5614 P&E 5614 ICG 11/15/76 04/11/91 1
5406 CR 5726 CR 5927 EL 1407 ICG 12/13/76 04/11/91 1
5407 CR 5727 CR 5856 PC 5856 ICG 12/13/76 04/11/91 1
5408 CR 5728 CR 5944 EL 1214 ICG 12/15/76 04/11/91 1
5409 ....... CR 5622 P&E 5622 ICG 07/19/78 04/11/91
5410 ....... CR 5624 P&E 5624 ICG 07/29/78 04/11/91
5411 ....... CR 5744 PC 5744 ICG 08/28/78 04/11/91
5412 ....... CR 5804 PC 5804 ICG 09/15/78 04/11/91
5413 ....... CR 5813 RDG 623 ICG 11/04/78 04/11/91
---- ------- ------- ------- --- -------- --------
5428 ....... CR 5874 PC 5874 ICG 08/22/78 04/11/91
5429 ....... CR 5879 PC 5879 ICG 09/22/78 04/11/91
5430 ....... CR 5860 PC 5860 RI 10/03/78 04/11/91
5431 ....... CR 5861 PC 5861 RI 12/30/78 04/11/91
5432 ....... CR 5750 PC 5770 RI 12/07/78 ........ 2
5433 ....... CR 5639 PC 5639 RI 12/05/78 04/11/91
5434 ....... CR 5631 PC 5631 RI 11/27/78 04/11/91
5435 ....... CR 5662 PC 5662 RI 11/17/78 04/11/91
5436 ....... CR 5621 P&E 5621 RI 02/01/79 04/11/91
5437 ....... CR 5618 P&E 5618 RI 01/19/79 04/11/91
5438 ....... CR 5615 P&E 5615 RI 01/26/79 04/11/91
5439 ....... CR 5835 RDG 633 RI 10/26/78 04/11/91
5440 ....... CR 5786 PC 5786 RI 10/14/78 04/11/91
5441 ....... CR 5660 PC 5660 RI 10/06/78 04/11/91
5442 ....... CR 5962 EL 1224 RI 09/19/78 04/11/91
5443 ....... CR 5961 EL 1223 RI 12/12/78 04/11/91
5444 ....... CR 5946 EL 1216 RI 12/23/78 04/11/91
5445 ....... CR 5941 EL 1211 RI 11/14/78 04/11/91
5446 ....... CR 5939 EL 1209 RI 11/14/78 04/11/91
5447 ....... CR 5937 EL 1207 RI 12/19/78 04/11/91
5448 ....... CR 5936 EL 1206 RI 09/10/78 04/11/91
5449 ....... CR 5625 P&E 5625 RI 09/10/78 04/11/91
5450 ....... CR 5607 PC 5607 MK 10/02/78 04/11/91
5451 ....... CR 5617 P&E 5617 MK 11/08/72 04/11/91
5452 ....... CR 5628 PC 5628 MK 10/24/78 04/11/91
5453 ....... CR 5912 PC 5912 MK 09/08/78 04/11/91
5454 ....... CR 5922 EL 1403 MK 10/27/78 04/11/91
5455 ....... CR 5932 EL 1202 MK 09/26/78 04/11/91
5456 ....... CR 5933 EL 1203 MK 10/18/78 04/11/91
5457 ....... CR 5934 EL 1204 MK 09/20/78 04/11/91
5458 ....... CR 5979 EL 1241 MK 11/14/78 04/11/91
5459 ....... CR 5930 EL 1200 MK 10/10/78 04/11/91
5460 ....... CR 5993 EL 1278 MK 11/02/78 04/11/91
5461 ....... CR 5991 EL 1276 MK 12/18/78 04/11/91
5462 ....... CR 5838 RDG 636 MK 12/06/78 04/11/91
Roster Notes:
1. Conrail renumbered CR 5720-5728 in 1978 to CR 5400-5408 so GP8's and GP10's could be in solid number blocks.
2. CR 5432 was retired before 1986, but I don't know exactly when.