Passenger vs Freight loco's

What is the difference between locomotives destined for freight vs passengers? Why do freight loco's using have two leading wheels as opposed to the norm of 4 for passenger loco's? The smaller drivers on freight loco's are built for torque while passenger ones were built for speed correct? What other differences are there?

No specific difference exists. If you're designing an engine that's supposed to go fast, you naturally would like to give it tall drivers. But lots of passenger engines had 73-inch drivers, and lots of freight engines had 69-inch drivers. Don't waste time trying to find a rule about driver size.

None of us fans knows how important it was for a 70-mph-plus engine to have a four-wheel leading truck-- all we know is they usually did. We have no idea whether LS&MS was crazy to replace 79-inch-driver 4-6-0s with 79-inch-driver 2-6-2s on their top trains.

Can't add much to that.
A minor detail - an engine intended for passenger service would certainly have been equipped with a train heating steam connection, where as one that would never see passenger service likely would not be so equipped. In either case it wouldnt be readily noticeable, just another hose next to the rear coupler.

Certainly there seems to have been a widespread belief that 4-wheel front ("pilot") trucks were better than 2-wheel at high speeds. The LS&MS (a New York Central subsidiary, which suggests professional management and access to good engineering talent) experimented, as noted above, with 2-6-2 passenger locomotives, but soon went for 4-6-2 instead.

Guiding truck technology did develop. Late steam locomotive designs featured better centering devices (complex arrangements of rollers, of carefully designed geometry, transferring weight from the locomotive frame to the truck and -- every action having a reaction -- allowing the truck to exert a force turning the rest of the locomotive into a curve) than earlier ones. If someone who really understood the engineering told me that a late (late 1930s or 1940s) 2-wheel truck was as effective at high speeds as an early 20th C 4-wheel truck, I'd be willing to believe it!

Certainly the Nickel Plate operated its 2-8-4 fast freight locomotives at speeds greater than many railroad's passenger trains: there are stories of the Nickel Plate's fast freights pacing New York Central passenger trains where the two railroads' tracks run side by side!

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As for steam connections, etc, on passenger power: I think some railroads equipped some members of classes of fast freight locomotive with steam lines so that if needed they could be run on passenger trains. I think I remember reading (in John Rehor's book about the Nickel Plate) that one of the reasons the NYC&StL bought its passenger diesels (several years before starting to dieselize their freight operations) was because the Berkshires were needed for freight, and by buying new passenger power they could ensure that the passenger department didn't "borrow" the Berkshires!

Well,
it's simple:
Passenger locos had taller drive wheels. Because having fast speeds to go, the rotation of the wheels could not be set over some limits. This depends on the bearings and thus the crank pin setup.
So tall wheels and a close to center crank pin reduces the rounds per mile, thus the rpm for higher speeds is reduced - and thus the piston has to travel lass fast the distances in the cylinder, so lower cycles appear, making the entry and exhaust of steam easier to handle.
That's why you see huge express engines having large diameter drive wheels.

On the lower diameter freight engines the crank is more to the outside of the wheel, and so you get a better crank angle and lever - thus more power is generated and the traction significantly increased.
A challenger runs with 1750 mm drivers as fast as a bavarian S3/6 - about 120 km/h.... but the engine of the challenger is a freight setup, thus: More traction, more rotation, more power... even two S3/6 won't match up one Challenger
The 844 hat a 2000 mm driver, thus will hold the 120 km/h (74,5 mph) much better and with less rpm than the challenger, thus the boiler hasn't the delivery need of so much steam, so is more economically - and you have a little of sprint abilities left... thus in 120 km/h the piston of the Challenger has to travel faster and making more distance, than those of the 844...
This is serious, because to piston represents an important mass going forwards and backwards, resulting in a swinging, wave like motion of the engine... Results are waving and wanking of the engines, disliked by most designers... and as more revolutions you have and as more powerful the gear is - like in freight locos - as more those motions disturb the smooth glide of the engine. Also the larger wheels give a more and better track stability, because exhibitin a better gyroscopic stability, that the smaller wheels.
Also the faster piston motion is more difficult to handle, because exhaust and entry timing are more difficult and the lubrication can get tricky...

But:
We can simply say, that some of the modern northerns, with their 4-8-4 wheel set made the best all purpose locomotives, exhibiting good speeds and good tractions power, because of four driver axles and usually a good speeding ability.

Wheel, the leading truck or leading axle depends on how this is arranged. For most freight train locomotives, the engine does not have those fast speeds to go, so a leading truck isn't necessary and later modern steering arrangements, like the Krauss-Helmholtz steering axle, gave a very good ability on higher speeds and even narrow curves, without having the engines to get in traction powers in curves.
In the former Bissel axle, the leading wheels shift the frame only, thus the first drive wheel runs very roughly on the inside of the track, resulting in high forces and a minimation of tractive efforts on this axle - this was avoided by the Krauss-Helmholtz steering.
Locomotives with many driving wheels in Germany used later the Schwartzkopff-Eckhardt-steering gear, which was an improvement of the Krauss-Helmholtz steering gear... So the later came more with freight and narrow gauge steam engines - the Krauss-Helmholtz on passanger locomotives.
Express engines used the leading trucks, because the truck gave a more smooth control in curves, especially on high speeds. Thus the leading and steering of the frame in curves was taken by the first axle of the truck, shifting the truck and giving a good steering force to the frame, so here steering gear were not need, also the trucks exhibit the ability to include extra air brake cylinders, increasing the brake power of the engines, important on high speed safety reasons. Also the leading truck is much better for weight purpose, as the cylinder and front boiler loads can be better taken by the trucks, as for an single axle.

Thus... construction of a steam engine depends on what you want to do with... and a sportscar isn't a car a ranger will choose for his duty... and a cross country car isn't a choice for a racecar driver to go on the track.
So you design the locomotive for the purpose - and for this you choose axle arrangement and leading truck/axle set as well as wheel diameter...

Even today, there is no one for all purpose locomotive, even it's more easy with modern electric traction, to get close to this "one locomotive all purpose"....

Steffen--
Do the Schwartzkopf-Eckhardt and Krauss-Helmholtz arrangements rotate the axis of the driving axle relative to the locomotive frame? This would make the "rigid" wheelbase-- the set of driving axles-- into a sort of "radial truck": for very high tractive forces even the (notoriously conservative!) American railroads have recognized that "radial" trucks, in which the driving axles in a single truck (bogie) are not permanently parallel to each other but can be rotated so each axle is roughly aligned with the radius of a curve in the track, are desirable for C-C diesels... even though the wheelbase of a single three-axle truck is much shorter than the "rigid" wheelbase (= wheelbase of the driving axles) of, say, a large 4-8-4.
What late American steam locomotives had was "lateral motion" motion devices on one or more of the driving axles. These did not give a radial effect -- the driving axles were kept perpendicular to the frame -- but it allowed them to move side-to-side, with a careful calculated resistance force, so the leading and trailing drivers wouldn't be "biting into" the rails on curves as much as they would be if they were kept fixed, and the resistance force meant that they would supplement the pilot truck in steering the frame into the curve.
To cite a typical example: according to the article on the New York Central Railroad's S1a "Niagara" (a large 4-8-4 with a total driving wheelbase of 246 inches) published when they were new (*), the first driving axle had 5/8 inch lateral motion on each side: the axle could, going into a curve, slide about 16 millimetres out from its normal position. Most big, late, American 4-8-4 had comparable lateral motion on one of more driving axles.
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(*)"Railway Mechanical Engineer" for October 1945: reprinted in "Train Shed Cyclopedia #56" (Newton K. Gregg, publisher: 1977)

Allen,
no rotation, it's a sideways shift, as the axles had some space to move.
If you have 5 drive axles, or 10 drive wheels you will encounter problems in curves, because of to narrow radius. For example, curve to left: The first wheel flange getting in contact is the first drive wheel right. But in the curce, the last right wheel flange will maybe also get in contact with the track, thus both flanges try to push the engine - in opposite directs. The rear flange pushes the frame to the right, the front flange steers the frame into the curve to the left.
So in such cases a derail can occur.
So you give the drive axles a little space to move sideways in the axle boxes, this enhances the curve ability. In our example now the first axle is pushed to the left by the track touching the flange on the front right wheel, and a little later the axles pushes the whole frame to the left, to steer the locomotive into the curve. Because of this space, the rear right flange doesn't get in contact with the outer rail, and the steering to in narrow curves is enhanced.
Problem is now: The locomotive will start to swing on straight tracks, because the space will lead to swinging motions of the locomotive front, which can cause derailment on high speeds - so such spacing in sideways motions of the axles will result in reduced top speeds.

So you have the front axle, this leads now the engine into the curve, pushing the frame into the curve, in our example to the left... this will decrease the impact of the rail to the front right flange of the front right drive wheel... thus the engine can go higher speeds.
To the Krauss-Helmholtz-Steering is a lever system of the front leading axle and the front drive axle. So here the leading axle is pushed to the left, because the outer rail forces the front right flange of the leading wheel to shift to left. The long Krauss-Helmholtz-Lever now pushes the front driver axle to right, making the front drive axle get in contact with the outer rail and the right wheel flange, so as the engine goes further into the curve, the frame is smoothly pushed to the left, so because having two flanges in contact with the outer rail, the whole frame is pushed nicely into the curve.
The later Schwartzkopff-Eckhardt-gear was a double steering lever, the front axle controled the second drive axle, and from there a second lever controlled the sideways motion of the first front drive axle....

Do you understand? These steering gears were usually constructed for tank engines with many drive axles, but the Krauss-Helmholtz-Gear made it also for fast going passenger engines with 2-6-2 wheel set. The express locos still kept the leading truck, because of the better steering motion to the main frame, and the better leading abilities on high speed, that's why you have the 4-8-4 as superior steam engines world wide...

And this is a steering gear, and other axles might have additional sideways spacing ability, for better curve adaptation.

Best google for "Krauss-Helmholtz-Lenkgestell", for a drawing, it will explain the function much better, than my improper english description.