BN: Almost EVERY BN bought or owned unit had a Leslie S-3K or RS-3K mounted, until the late 80's when the SD60's and 70's came along, they had K3LA's.
SCL: Almost every unit owned had a Leslie RS-5T horn.
N&W: Most units had Nathan-Airchime P5's or Leslie RS-5T-F's.
WM: Leslie R-S5T's.
GM&O: Most had Leslie RS-5T-R25 horns.
Conrail: Most owned units had Leslie RS-3L's or Other variants of said horn.
Amtrak: SDP40F's, Leslie SL-4T's. F40PH's, Early versions had Nathan P5's, later had Nathan K5LA's. Genesis series: Nathan K5LA/K5L and Various Messed up Nathan Productions. F59PHI's, K5LA.
As always, there are exceptions. Some BNSF units are known for having P3's and P5's.
Here's a breakdown of the common stuff of today:
CSX Transportation:
-Most GE Power has a Nathan K5LA, K5LAR24, or K5L/K5H. (The difference in the K5L/K5H is the manifold height.) There's various ex-Conrail stuff out there that might have a Leslie S3 or something of the like.
-Older EMD power, such as an SD40 or something, may have any number of things, but is usually limited to a K5LA or older Leslie, such as an S3L.
-Semi-modern power, such as the SD70M, has a K5LA, and SD70ACe's have the routine K5LLA.
BNSF
-MOST Dash-9s are equipped with a K3L or K3LA, but there's a small handful out there with K5LA's. Some might even have a Leslie RS3L, but you'll have to look around for those.
-ES44AC's/ES44DC's/ES44C4's are typically equipped with a K5HLL, but the earlier batches have been known to have K5HL's, and, again, a handful have K5LA's.
-SD70ACe's seldom have a regular K5LLA, as the newer batch that BNSF has is known for a K5LLAR1L.
-Older EMD power typically has a K3LA, but you'll occasionally get lucky and find an ex-ATSF unit with a Leslie RS3L or S3L on it. Same goes with semi-modern stuff, such as SD75M's.
Union Pacific:
-Modern GE Power goes the same way as BNSF; most newer ES44AC's/ES44DC's have a K5HLL, but the older batches have K5HL's. Again, there's a small handful out there with K5LA's. Now, in the way of Dash-9's/AC4400CW's, most have a K3LA, but you'll find some ex-SP around with a Nathan P3, and there's a handful out there with a K5H/L.
-SD70ACe's have K5LLA's, but, as I recall, the new, isolated-cab batch has the K5LLAR1L. Some do have K3LA's.
-Semi-modern EMD power, such as SD70M's, typically have K3LA's, but there's even a handful of K5LA's and P3's in there.
-Older EMD power, such as the SD40-2's, has an almost even mix of K3LA's and P3's, in that a good bunch of those are ex-SP units.
Norfolk Southern:
-Modern GE power tends to follow the CSX routine, in that they use K5LA's, and K5LAR24's, an even mix of the two. There's even some Dash-8's/9's that feature P5's. And, again, some of the older Conrail stuff has either a K5H/L, or maybe even a Leslie.
-Modern EMD/Semi-modern EMD power tends to have the K5LA/K5LAR24 on it, but, again, there's some stuff like the standard, spartan-cab SD70, or even an SD70ACe that has a P5 on it.
-Older EMD power tends to have a P5 or Leslie on it, such as ex-Conrail stuff.
Amtrak:
Anthony nailed it on the head.
-Modern GE stuff, such as P42's, and P32BWH's, have K5LA's, with a few K5L's and "hybrid" K5LA's thrown in. There's been an occasion where an M5 was tossed on a Dash-8.
-Modern EMD power, such as the F59PHI's, have the same stuff. The complementary cabcars have the same. Although, there is a P5 on an AmCal cabcar down in the bay area.
-Cabbages have K5LA's, excepting one or two out there, one with an original P5a (Heartland Flyer service), and one in Cascades service.
-Switching power tends to have an older Leslie, but there's a mix of K5LA's on units such as the GP15D's and CF7's.
