Leased power could appear depending on what line you were on and who interchanged at the end of it.
1990s, I personally remember seeing on the Chicago Line in New York:
TV556 would almost always have SP power that could be anything on SP from their newest GE's to 60's GP40s to leased MK ex-ATSF F45s and SD45s. SP units were otherwise fairly rare.
The TV that was handed to the NYS&W for a time at Syracuse usually had BN or BNSF units of some sort going east, again occasionally with leasers. The BN units would run right down to Geddes St. to cut off, then run light to Dewitt and normally got sent west on something else. Until then, BN power was very unusual.
UP units were very common and could show up anywhere, there are even pictures of one on the former Lehigh Valley on a coal train up through Ithaca.
Santa Fe units were less common than UP, but did appear now and then.
NS units could not lead per crew union rules because they lacked chemical toilets and built in coolers, normally when I saw them a Conrail was tacked on the point. Newer NS units might lead, and once in a while one would lead on coal trains when the Hudson River trains were routed via Buffalo. A Santa Fe showed up on one of these trains, with the NS units.
CSX units were fairly common, usually in singles; the coal train for Solvay NY had them regularly until the contract went to a mine on NS. That included at least one road slug set, and I saw a couple of units still in Family Lines paint.
CN units showed every day once CNSE and SECN became runthroughs, but only between Selkirk and just west of Syracuse. Very uncommon anywhere else, but it's entirely possible a set could have been used to run down the River Line now and then.
CP units showed once in a while on unit grain trains. Otherwise uncommon. Grain trains went to Albany I believe. This could include Soo units, also.
A coal train that ran once a week or so tops on the Montreal Secondary, towards the end, would often show with WC units. Otherwise they were rare.
IC units were very uncommon and unpredictable, they're more common since CN bought IC but still unusual.
CNW units were unusual, but more common for a time after UP took over. The newer Dash-8s are what I remember most of, but I did see a few SD40s.
Rio Grande units were like IC - unusual - but did show now and then.
Guilford units could run through, but were rare and usually paying off horsepower hours - you'd see the same one two or three times in a week then it would dissapear.
I don't think I ever personally saw a KCS unit up here.
Other oddballs: New England Central units in transit; Housatonic units in transit; MA&N units in transit; MNCR FL9s in transit (those were easy to pick up in a train, as they'd cause an odd axle count total on the detectors).
NYS&W units I don't remember seeing mixed with Conrail. They got out in TV trains to and from NS at Buffalo and I'm sure were handled to Rome Locomotive Works when that was in business. Sometimes trains off the NYS&W were left on the siding at Belle Isle, about 10 guys chased that move out to CP296 and back with a new SD70 on the west end if I remember right.
Leased units could turn up but usually with foreign power - Conrail had enough units to play with their own lease fleets by then. I remember the UP-painted GECX units, M-K units, one Helm unit, and others.
For a time D&H ran detour trains Albany-Buffalo, so you got all of their stuff up here too - the NRE SD40s, CP/Soo units including a few ex-MILW "bandits", GP38s, and some other junk. I don't remember CP Alco/MLWs and by then the GP39's were gone.
Anything on Conrail could appear up here; I recall one power set that was an SD45-2, an NS unit, and a CSX unit. I saw ex-PRSL units, one ex-EL GP35, 3 of the 4 LV GP38ACs, 4 out of 5 ex-RDG GP40s, and I think all 8 ex-CNJ SD40s. Even units as old as GP10s turned up as long as Conrail owned them. Switchers were rare and usually in transit, and dead units were rare - one whitelined GP30 is all I ever caught, although I did see the 6607 smoking like a steam engine one day and read not long later in Railpace it had been taken off the roster. Apparently it was still blowing up.
So if you don't want to have 1000 units, have 2 or 3 UP and one NS, CSX, or whatever else you happen to like the looks of and you should have a reasonable variety.