It seems that ALL highpowered B-B units are very slippery by nature. (Why I asked about wether any B-B AC drive units were in use) The above posts should be food for thought.
Never forget that putting power in the railhead is the name of the game. All locomotive operators need to win that game to be cost effective. I would not want to be the exec at BNSF in charge of this program.
Maybe if someone who has run a say Dash 9 (a DC model) and then ran a Evo in AC trim would share a good story as to the differences in adhesion/power/traction on roughly the same loco wiht the same HP, with the variable being AC compared to DC, and if the AC models are more slippery than their DC counterparts with heavy freight locos.
In my own personal experience in passenger, with both electric and diesel DC and AC- ALP 44, which is DC, and ALP 46, which is AC - that the slipperyness is MUCH more on the AC motor than the DC. I have always figured its the way that the power is being put to the rail, in that the power being used by the traction motors tends to decrease as speeds increase with DC (at least in Amps), while the AC tends to hold steady/increase as the speeds increase. (the needle all the way up at the "power" line)
This is only a "seat of the pants" and "by the eyes" observation, of actually running the locos and watching the load/ammeters.
Same with the diesels too - a Geep (DC) will load less as it is accelorating, and when it gets up to 80 plus MPH, in notch 8, its only drawing 300-400 amps at the traction motor. While a PL42 will keep building power and at 80mph under full throttle its putting 100 percent power to the wheels.
It seems to me that there are two types of wheel slip that B-B AC powered locos deal with - starting off - tractive effort, and then wheel creep and then slip, while at speed, or accelerating. Ive noticed at least on the PL42s and the ALP46s that it happens about 53 or so MPH. Where on a DC powered Geep, (or even ALP44 DC electric loco) that that wheel slip/creep at speed happens much less often, and usually only under bad rail conditions, namely because the TMs are getting less current.
I apologize as im not real good with the technical info as to what it does why it does it, I have forgotten and purged all that from my brain
but I just know what the loco does and what I have noticed might cause it, and how to deal with it.
But getting back to these freight units, I wonder if the slipperyness of the ACs in 4 axle trim in passenger service will spill over to freight in the A1As, and I can see that being a problem if you are sailing along at speed on a nice, high speed intermodal train, uphill and in the rain, and then the loco gets some nice wheel creep and then slip (and then regains traction) around 50 or 60mph. Can you say pulled drawbar?