Okay, mind me rehashing things as I get a foothold on this thread and get my mind organized.
From Penn Station north, we have the following power options and tunnel/Bridge layouts (per Wikipedia and Wikimapia):
- Penn Station tunnel under Hudson Yards from station to 36th street. (West line starts)
- A tunnel between 39th and 43rd
- Overhead catenary and top-contact third rail (LIRR style), NYP to 41st street.
- A short tunnel 44th to 45th.
- Freedom Tunnel 45th to 123rd.
- Open air until Inwood/Spuyten Duyvil Bridge.
- The bridge, which is a swing-arm.
- Bottom-contract third rail (MNRR style) Riverdale (end of West Line) to Croton-Harmon (assume all tracks although diesels run express on inner tracks).
- No power north of Croton-Harmon
Hmmm... I'm reminded of the New Haven line and that section of the NEC...
- From Grand Central to Pelham, third rail MNRR style.
- Overlapping catenary (12.5kV 60Hz) for a quarter mile south of Pelham.
- Catenary all the way up to New Haven, and past to Mill River.
- Newer caternary (25kV 60Hz) Mill River to Boston.
Aaaah... MNRR's M8's swap between catenary and third rail. I though it was Amtrak that had to do that. Amtrak just switches between 12kV 25Hz (PRR) and the 60Hz systems like nothing. Also, Amtrak swaps engines in Albany, no matter what. Their P32AC-DMs are restricted to Albany to Penn Station. And finally, the Inwood/Spuyten Duyvil Bridge opens up for barge traffic!
Now we see what the technical problems are. You can electrify the West Line via catenary but you're stuck w/PRR catenary until Amtrak remodulates to 60Hz throughout the NEC. You got the bridge to contend with. You have third rail but it's not quite expensive for MNRR to switch to 60Hz catenary at Riverdale, and run that all up to Albany (the M8's handle that).
It's the freakin' center pivot swing-arm bridge at Inwood! You have to get power to it, because any catenary needs to be powered right then and there.