I have seen 6-axle electrics -- I rode behind one once on the OBB (Austrian Federal Railway).
Amtrak famously purchased 6-axle electrics from GE, and they seemed to fall off the tracks at the speeds Amtrak wanted to operate them.
The reason for 4-axles on an electric locomotive may have to do with the only demand for them being Amtrak in high-speed passenger service and that 2-axle trucks of current designs may be more stable at high speeds than 3-axle trucks.
As to the horsepower, what the electric wire gives you is the ability to tap into nearly limitless power (if you are willing to pay the electric bill) whereas a Diesel-electric incurs the weight penalty of the the Diesel engine and its alternator to power the traction motors. Yes, electrics typically require transformers and other electric gear, but there has been technological progress making that lighter in weight.
The faster you go, the more HP you need for the same tractive effort and also the more tractive effort you need to overcome aerodynamic drag. Electric locomotives are offered as the solution to getting enough HP without a high weight penalty to power high-speed trains.
You do give up starting tractive effort -- one is from the axle-load restrictions so high-speed operation doesn't pound up the track, another is that your traction motors need "tall" gearing to run at 150 MPH+ without "birdnesting" the traction motors (that is, having them fly apart into a messy mass of copper and iron parts). AC traction may help with that because the rotating part is much simpler and rugged.
The other thing is that "back in the day" it was thought you needed all axles powered for HSR, which is what the Bullet train does. Part of that is that the Japanese HSR is a mountain railroad, but part of that was the thinking that adhesion, or the adhesion you could rely upon diminished with speed. The newer fast-acting wheel slip controls, apparently, allow the use of locomotives for HSR, or it least in European practice.
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
Like most people, you haven't figured out what power is.
It's force times speed.
For a locomotive, that might mean 50000 pounds of pulling force (at the wheelrims, on the rail) times 45 miles per hour (if the locomotive happens to be running at that speed) is 2,250,000 units of power. Nothing wrong with those "units of power", but as it happens no one uses them; instead, we say 2,250,000 units of power equals 6000 horsepower, an arbitrary definition.
The point is, at constant power, pulling force must double if speed is halved. If that locomotive is producing a constant 6000 hp at the wheelrims as its speed drops, it has 100000 lb of tractive effort at 22.5 mph, and 200000 lb at 11.25 mph, and 1000000 lb at 2.25 mph.
You can see the problem-- no locomotive can maintain constant power down to a standstill. The wheels will eventually slip.
Unless you pile more weight on the wheels, which is what diesel locomotives do. To reliably exert, say, 150000 lb of tractive effort, a locomotive needs to weigh more than 400000 lb. So it needs more wheels.
In other words, the four-axle electric puts out more power than the six-axle diesel at 60 mph, but much less power at 10 mph.
I really can't wrap my head around this, google is no help. Can someone who knows the answer please explain?
if diesels have half the horsepower, why do they need more axles to put half the power on the rail ?!?!?!
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