For those interested here's the proposed service plan for GO RER. Seems it will run a combination of EMU, Electric Loco, and Diesel Loco. Pic related is Peak hour service.
If you want more detail heres the full business case (its a PDF):
http://www.metrolinx.com/en/docs/pdf/board_agenda/20181206/20181206_BoardMtg_GO_Expansion_Full_Business_Case.PDF