Putting aside being able to put up the poles before any other construction, there is no way it should take more than 3 years to electrify 250km of track.
Are we talking about 250 route-km of track, or 250 actual miles of trackage on somewhat less corridor length? Because those are two completely different things.
As for whether it will or won't, a lot of that comes down to preparation. The Brits installed 1400 track-km of overhead on the East Coast Mainline in the 7 years from 1984 to 1991 - but that didn't include the year or so of prep and engineering ahead of that, and it also resulted in a system that is now having to be upgraded in some places and replaced in others. Be careful of what you wish for if speed is your priority.
All they need due is contract the construction to several different companies at the same time. As for not having any experience, that is a loud of crap. As I understand it, subdivisions get new lighting and electrical poles put up all the time all over the damn place.
You realize that the number of contractors that are capable of working near operating rail lines is extremely limited, right? Hell, most of them are already contracted to do other work for Metrolinx.
And how many subdivisions are required to keep trains running past at 60 or 70mph every 15 minutes? The comparison is a laughable one at best.