Pretty hard to get agreement to spend ma$$ive amounts to upgrade something that’s working well enough.
The 25HZ system worked well for the Standard Railroad of the World (Pennsylvania RR) and Penn Central, Conrail, and Amtrak chose not to do anything about that. Mostly because it worked.
What is now the Corridor North of NYC to New Haven CT was ex-New Haven RR and ran on 11KV 25 HZ from the Cos Cob power plant. North of New Haven to Boston was non-electrified until Amtrak installed the present 25 KV 60 HZ system. I thought I remembered reading that when the Cos Cob plant was phased out, the catenary once fed from it was converted to 60HZ but I haven't been able to find that info other than a note that shows the Amtrak Hell Gate line (which would be the ex-New Haven North of Bowrey Bay) is 12kv 60hz.
Amtrak's 25 Hz traction power system
Amtrak's 60 Hz traction power system
Thus in traversing the NE Corridor from Wash DC to Boston you are running on at least three different electrical systems of 11, 12, or 25 kv and 25 or 60 hz..
Whether or not a future replacement of any of the generating capacity that the PRR installed causes an 'update' to 60 hz remains to be seen.
The PRR power sources were a bit more reliable than the New Haven's Cos Cob facility.. ;-)
Now if Conrail had chosen to repair the Poughkeepsie Bridge after the fire in the '70's the railroad scene where I live would be a lot different now. But that has nothing to do with this discussion. ;-)