Based on a Transit Toronto article I read (
http://transit.tor...transit/2509.shtml), GO probably won't be in the market for a new locomotive for quite some time (another 10 years probably). When you think about it, most of GO's prior locomotives (F40PH, GP40 variants) were older than the F59PH is now or at least AS old. The reason GO isn't on the hurry to replace the F59PH is that it is so much more technologically advanced than the previous fleets. It incorporates an in-engine HEP system, dynamic braking to 6-8km/h, and advanced computer micro-management systems that can still be updated to todays standards.
However, if GO were to replace them they wouldn't have too much of a choice right now. Most other commuter agencies across North America are still using F40PH's (Tri-Rail) or F59PHI's (Coaster)... theres also that other new EMD locomotive that I think LIRR is using. It looks butt ugly, I think its called MP36PH or something. Hell, VIA and Amtrak are still using F40PH's too.
If anything does come out in the future with a commuter thought of design, it will probably come from GE... EMD isn't doing all that great these days.
BTW, what are these mechanical headaches you speak of? As my post suggests, all I've heard about the F59PH to date has been good.