<i>As for re-building the central:
1. The 3rd track will also be very expensive, maybe even more expensive. It has to deal with building in areas that are fairly narrow. Grade crossing reconstruction also. Either way, I think that if the chance to have new passengers and a LOT of extra capacity is present, they should take it. Bridges? The only bridges I can thing they'd need is over the Meadowbrook Parkway and then over the tracks where they meet at B tower. </i>
No doubt the 3rd track will be $$$, but, a central reactivation will be MORE. not only because of the legal stuff (millions alone), but the 'studies'. And, there'd be massive 'dirt work' required. Very expensive.
2. The fact that the rails haven't been there really is no problem, IMO. The ROW is there, it would serve the community greatly, and the NIMBYs that whine about it can kiss my ass for all I care, b/c a few monthes after service starts, they'll be riding those trains. </i>
Like it or not, it IS a problem. For the last 50 years, the ROW hasn't even come close to resembling a rail line. In most places, there are NO tracks, or even any hint a rail line existed there. this isn't a case of homeowners moving in next to a deactivated line, there WAS NO RAIL LINE when they moved in. It didn't exist. It wasn't 'there first'.
3. Why not as a diesel line? It'd be cheaper to build and maintain. </i>
Because it won't add much capacity to the system, and by the time you're done with everything else, the costs of electrification will be nothing. We're easily talking about 100+ million a mile here. At that point, the cost of third rail is not much Vs the added advantages it'll have. In any case, have fun selling a rebuilding of a 50 year gone ROW that abutts hundreds of homes, just to run diesels on it. In today's political climate, it's not going to happen, and even MU operation would be viewed as 'too loud' for some. The added value - the ability to reroute Ronkonkoma line trains to it, lower noise, would far outweigh the minimal added costs, when you look at the overall picture. And it'll only have to be done in the future anyway.
4. Hempstead line really wouldn't need much upgrading. The main line would, but it has needed upgrading(double tracking) for a long time now. And even if the trains can't run on the main, they can go down the central to Babylon, maybe even PD and reopen the south farmingdale stop </i>
You'd want higher speeds west of Garden City. As for the Ronkonkoma branch, it could use double tracking, but that's another ball of wax and another big outlay.
Like I said, realistically, you're talking about a billion plus dollar project. probbably twice that amount. It's effectively 'new' track construction in a very crowded area, with lots of special needs (ASC, better grade crossings, high platform stations, bi directional signalling, overpasses over roads that can't be closed durring construction, tight clearences).
Look, I think it's a good idea, too, but right now, it's simply not that practical, because of the cost / hassle to do it. 20 years from now, when a third track's in, it's 2 track to KO and beyond, and the system's capacity is maxed out, and demand is still growing, it'll make sense to do it. But now, a 3rd track is cheaper and more viable.