High Hood,
I dug out my books, which indicate:
Seaboard System (as of 1/1983): 54 SD35s* (road nos. 4505-4548, 4584-4593)
Seaboard System (as of 1986): 56 SD35s* (road nos. 2421, 2429, 4511-4548, 4584-4599)
Chessie System (B&O) (as of 1/1976): 24 SD35s (road nos. 7400-7419, 7437-7440)
Chessie System (C&O) (as of 1/1976): 12 SD35s (road nos. 7420-7431)
Chessie System (WM) (as of 1/1976): 5 SD35s (road nos. 7432-7436)
Chessie System (B&O) (as of 1/1987): 14 SD35s (road nos. 7402, 7407-7419)
Chessie System (C&O) (as of 1/1987): 9 SD35s (road nos. 7420-7421, 7423-7426, 7428, 7430-7431)
Chessie System (WM) (as of 1/1987): 3 SD35s (road nos. 7433-7435)
FYI, Atlas will be producing N scale SD35s in Seaboard System, Chessie System (B&O), and Chessie System (C&O) later this year. Check it out:
http://www.atlasrr.com/NLoco/nsd35c.htm* While I tried to disregard the SDP35 model, which was inherited by SBD from SCL, my source also indicates that the SD35s numbered 4524-4526 and 4534-4535 are some of the former SDP35s)
Hope this helps,
DFF