<i>As for Oyster Bay, why does it have to be electrified to have potential? </i>
Because it's too *explitive deleted* slow as a diesel line, people hate changing trains, and the DM's don't work very well. On that last note - I saw a 6 car train with 3 500 series locomotives entireing the line Friday. WTF was up with that?
Really, the OB line just doesn't work as a diesel line. Where the track is straight, the stations are too close for the diesels to get to any decent speed, where it's hilly, it's too hilly for them to get to any decent speed, where it's curvy, it's hilly anyway.
As a kid, I used to pedal to Sea Cliff, watch a train go through towards OB, then go down to Glen Street and watch the same train leave Glen Street. And this wasn't hard to do at all.
Anyway, from Rosyln to East Williston, it's practically straight and could be 80mph, but the diesels are too slow to get up to that speed bewtween stops, from Greenvale to Sea Cliff, ditto (well, almost, there's that curve by Sea Cliff). The only real slowdowns on the line are Roslyn to Greenvale, and Sea Cliff to Glen Cove, and that part beyond Locust Valley that nobody rides on anyway. The former wouldn't be helped by MU operation, the latter probbbly WOULD. Electrics simply outperform diesels on grades, period.
On top of that, direct service to GCT and FBA could be had, something that's impossible with the existing equipment.