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Another perspective on the answers here... Steam switchers usually had only coupled wheels in order to put all of the loco's weight into the tractive effort. (When non-driven wheels are added they reduce the tractive effort by the amount of weight they carry... so why have them? At the front end it mostly relates to easing transition into curves and also supports some of the front end weight of the design - but some of the front end weight is put onto the leading wheels to ensure they stay on the track... but, contra to what might be inferred from one of the above, leading wheels do not make something more that can be derailed... okay they tend to derail more often than drivers.. that's because they get there first. Trailing non-driven wheels usually result from boiler and firebox size and design... without a trailing axle or two there would be too much overhang and an imbalance of weight to the rear of the drivers... why not more drivers to support the back end? Some firebox designs don't leave space for more drivers. There is a limit to the number of drivers a combination af boiler/cylinders can usefully drive. There is a limit to the number of drivers that can work in a rigid frame on curves of different radii. The other alternative is to go for an articulated loco... where the designers think this is appropriate... even there you often end up with leading and trailing non-powered axles) Meanwhile ... back at switchers etc... If you look at steamers you will see that as a general rule there were four classes of loco. Switchers, Freight, Mixed Traffic and Express. Switchers had least non-driven axles and (generally) the smallest drivers. They could be pretty massive (I think 0-10-0 was the biggest non-artic) but they were designed for shifting heavy weight slowly. They also rarely went far from their home yard and fuel water supply. Freight, from local to long distance heavy drag came next in wheel size. Locals tneded to have leading and trailing trucks so that they didn't have to be turned on long spurs. Long haul would turn or Y between trips. Both increased in size to fit their home range/division. Mixed Traffic (more rare in the US I believe) came between freight and express - I use the word express rather than passenger because there were many smaller branch and suburban passengers that mix things up... and because some freight... reefers especially ran express - A mixed Traffic would haul perishables, milk, mail and reefers but could be bumped up to passenger express for Thanksgiving and other heavy holiday traffic periods. Express. Not the biggest (that was the big freight) but usually the smartest with the biggest wheels for speed and frame/truck design for getting round the curves smoothly at speed. These were the least good at small moves... like setting cars into a terminal station... they tended to eat fuel and water... so again you didn't want them messing about getting their train out of a carriage yard and hanging about in the terminal. Much better for a switcher to do the donkey work and have the thoroughbred drop onto the train at almost the last minute fully stocked for the journey. OKAY... SO DIESELS. people tend to design by what they know. So the first express diesels (that developed into the E8s and E9s) were streamlined throroughbreds. Suitable gearing for getting there in a hurry. It's interesting to notice that right from the start they were effectively "cab ahead" units despite the fact that early freight soon switched from the "cab ahead" F and similar units to long-hood forward hood units. I mention this because we tend to assume that diesels ran long-hood forward following the usual order of boiler-cab on a steamer.... but the Covered Wagons broke this convention right at the start. There was another advantage to the big passenger diesels... they could run through saving journey time taken by changing steamers. (For newbies steam engines were like stage-coach horses... they could only be run so far before they had to be serviced). I'm not an expert on what came where, when from whom but my first thoughts on this subject were that Alco distinguished between its "S" switchers, "RS" road switchers and Passenger classes (The DL109s were really fast passenger but called Mixed Traffic to get them by war time building restrictions - same thing happened in the UK with at least the Merchant Navy Class "Spam Cans"). I think that Road Switcher goes a long way to explain what went on and what has developed. The RSs and later GPs - General Purpose - were mules, oxen or medium carthorses. They could switch most yards, work on the main and work the lighter branch lines They were exactly general purpose for switching and road use. EMD decided that the big diesels with six axles were "SD" = Special Duties... hauling coal and ore... the big stuff needing muscle. Alco fought back with their Century series but GE pulled the master stroke by designating all their locos "U" = Universal... go anywhere, do anything. One basic breed of locos, one basic set of spares... Then things got BIGGER... so size for size an early SD now compares to one of the big AC things in the way that a GP used to compare to it. As has been noted there is still demand for the small switchers or small GPs. The locos not much mentioned here are the small industrials and critters. They still quietly do their bit where appropriate. Also, a completely different issue... passenger diesels have seperated off into a completely different line of development. Q2 People have covered where switchers appear. the simple answer is "where they are most suitable and available. Q3 Horse Power... Also answered.But it's not just raw horsepower... it's getting the power on the rail... so there have been devekopments in traction control and there are slugs. There are also DPUs (Distributed Power Units)... which also factor into the whole equation of what sort of loco to do the job. BUT Qs 2&3... what happens is a mix of what is available (including what cash is available), what is best suited ... and that means what power is needed, track conditions, loads and all sorts. The solution is not just switchers, GPs, SDs or modern big power but what will get the right amount of power down most cost effectively. If a job is best haued by a pair of ACs... say a unit coal train... it may also be appropriate for them to stay on the train for the slow crawl to load or unload on the move. Elsewhere it may be appropriate for a switcher to dawdle off along the main with a couple of cars. Then there's cow-calf combination, slugs and things... Hope this helps. Er... what I haven't said but others have commented on is that diesels get different wheel sizes to a small degree but lots of different gear ratios and turbos or not turbos. these don't show up in the same way that different sized drivers and wheel arrangements did on steamers. This led very rapidly to steam fans saying that diesels were boring because they all looked the same. The biggest give away is often in special trucks... and that gets knobbled by RRs trading in trucks... Fuel tank size and dynamic brakes make a difference... but never as much as between a heavy freight and a light passenger steamer.
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