There are currently 8 surviving Big Boys.
Here are the factors that really affected the design of a wheel arrangement:
The amount of weight on the drivers improved their pulling power. You typically want a ratio of 4:1 of weight on drivers to tractive effort. This was known as a factor of adhesion. The N&W class As were notoriously slippery engines due to having a low FA. They received a limited cutoff essentially to reduce their tractive effort in order to improve their factor of adhesion.
So, the more weight carried above the drivers, the better a locomotive would be at starting a train.
Since steam pressure was what powered the cylinders, it was always a good idea to be able to produce amble steam to fuel the cylinders. The faster a locomotive operated, the more steam its boiler would need to produce, and hence the larger the firebox it would need. Until Lima's A1, freight trains typically operated around 15mph as the 2 wheel trailing trucks couldn't support a large enough firebox to maintain higher speeds. The A1's revolutionary 4 wheel trailing truck design allowed it to produce more steam than the cylinders could use.
So, having a 4-wheel trailing truck really made fast freight possible, and hence it was a must for a large, fast engine.
The number of drivers was primarily a result of distributing the weight of a locomotive. The same weight spread out over 4 axles instead of 3 axles would generally be easier on the track...especially with short bridges...but would result in slightly higher maintenance.
The Lead truck was primarily their to give a smooth ride. I know specifically that later 2-wheel lead trucks were specifically designed for high speed, but prior to the concepts behind the high speed 1-axle lead truck, the 4-wheel lead truck was the only safe way of operating a locomotive at high speeds. The first two axles carry much of the force of going around curves. You'll have slightly higher maintenance on a 2-8-4 than a 4-8-4 due to the wear on the #1 drivers of a berk where as the northern would wear the lead truck. This was noticeable on one of the restored berkshires.
Keep in mind, the Big Boys were neither the biggest nor the most powerful locomotives. They were smaller than the C&O 2-6-6-6s, weaker than the DM&IR 2-8-8-4s and C&O 2-6-6-6s, but they were pretty much exactly what the UP wanted. (Btw, you'll have to do a little research on the 2-6-6-6 weight controversy if you want to see why they're bigger)
Breaking up a 4-16-4 into a 4-8-4 saved dramatically on the running gear weight and also allowed the negotiating of tighter curves through articulation. Not all x-x-x-xs were articulated. The Pensy's 4-4-4-4s and 6-4-4-6 had rigid frames...they just used chose to save on running gear weight.
The UP's first truly monstrous beast (and my fav UP power!) was their 4-12-2. It was purely a freight engine, but designed with a 4-wheel lead truck for flying across the plains. There is 1 survivor, and its at the LA fairgrounds.
The Challengers were dual service locomotives. They actually weren't as powerful as some 2-10-4s, but who cares? They were designed to assault Sherman Hill and were the largest locomotives to regularly be used in passenger service. They had lower axle loadings than other large articulateds, which helped them to be versatile engines.
The Big Boys, if memory services me, were enlarged Challengers. They were intended for fast freight, with Sherman Hill as its nemesis. They were the zenith of UP power, and rightly are as synonymous with the UP as anything could be. Its just as shame that they never received the two-tone gray of their passenger hauling cousins ;-)
As for excursion service...it isn't the maintenance or the fuel costs that makes it difficult to consider restoring one...its the restrictions of operating a 4-8-8-4. Even if it will fit through the clearances every where, its going to have many speed restrictions and test a number of bridges load restrictions. For reference...when a NKP berk would travel the same excursion route of an N&W J, the berk would have almost no limitations...but the J would have a 10 page essay's worth of restrictions due to clearance, axle weight, and such (the J's have axle loadings almost on par with Alleghenies). Engines are designed for specific portions of specific railroads...and those designed to the extremes are very difficult to take elsewhere. The only way that a DM&IR yellowstone, a UP Big Boy, a C&O Allegheny, an N&W class A, a UP Challenger, an N&W class J, or similar locomotives can be operated is with exceptional support from a railroad.
That being said...there was a deal around 7 years ago to restore a Big Boy for a Hollywood movie, but the deal fell through. It was really sad that it didn't happen. I would love to see a Big Boy run. Maybe something else will come along. The Big Boys are the ideal jumbo locomotives to restore as there are 8 left (useful for spare parts), they have someone what reasonable axle loadings, and it could be possible to get future UP management somewhat on board with such a marketing tool...if they don't have to deal with the headaches.