The vast majority of steam locomotives were "Rod Locomotives" or "Rod Engines" characterized by rods on the sides connecting the pistons in the steam cylinders to the drive wheels and interconnecting the multiple drive wheel axels. The photo below shows a small Civil War era rod locomotive.
During the late 1800s rod locomotives started to appear in the logging camps of the eastern US ---- with limited success. The back woods logging railroad presented a severe challenge to any engine as light gauge track was usually used over rough roadbed with steep grades and sharp turns. In some cases, logs rather than steel rails were used for track. The rod engine was ill suited for such poor roadbed because most the weight was concentrated over the short wheel base drivers, the turning radius was large and the action of the rods tended to tear out the curves.
The geared locomotive was developed to meet the unique challenges of the logging industry. A number of different designs were tried, three of which achieved major success with sales of over 1,000 each. An interesting aspect is that all three were manufactured near my home in central Ohio, one about 150 miles to the northwest in Lima, Ohio and the other two about 200 miles to the northeast in Erie and nearby Corry, Pennsylvania.
My view of the key features of these three successful designs are:
The most famous of the geared locomotives is probably the Lima Shay built in Lima, Ohio. A unique feature of the Shay is that the engine (i.e. the cylinders) are mounted vertically on the right side of the locomotive with the power transferred to each axel via drive shafts and bevel gears. Because the engine and drive train are on the right side, the boiler is located off center to the left to balance the weight distribution. Having the engine on the side permitted the addition of a third cylinder without upsetting the basic design. The following photo shows an 80 ton Shay built in 1905 and still running at Cass Scenic Railroad in West Virginia.
Much more information about the Shays is available at:
Another design developed at about the same time as the Shay was the Climax, manufactured in Cory, Pennsylvania, a short distance from Eire, Pennsylvania. Key distinguishing features of the Climax are cylinders mounted on the sides and all axels driven from drive shafts that run down the center of the locomotive. The cylinders on all except the very earliest models were mounted at an angle. The photo below shows a fairly small (~40 ton) two truck Climax still operating at a park in Durban, West Virginia.
The most unique feature of the Climax design is the skewed (or offset) bevel gears that couple the crankshaft to the drive shaft and then the drive shaft to the axels. The drive shaft crosses below the crankshaft and above the axels. I understand this is the only known commercial application of these unique gears. The sketch below from an early Climax Catalog shows the Climax truck.
The third successful geared locomotive design was the Heisler built in Eire, Pennsylvania. The Heisler and Climax plants were a mere 20 miles apart and I understand used some of the same suppliers. Photos from an early Heisler Catalog suggest that Heislers were used in both the mining and logging industries. The unique feature of the Heisler is a V type engine coupled to center drive shafts that couple to only one axel of each truck thus avoiding the need for the offset bevel gears of the Climax. The two axels of each truck are coupled with rods on the outside of the wheels. The photo below is of a 90 ton three truck Heisler that is still operational at Cass Scenic Railroad in West Virginia.
One might ask, which of these three competing designs is the best? The following is some marketing information from an early Heisler catalog. It is easy to find the not very subtle comparisons with the "off-center" Shay design and the excess use of "inefficient gears" in the Climax design
So, which do I think is the best? I guess I don't have to worry about best since I don't run a real railroad with real profit and loss concerns. I like them all, each for their unique features --- much like an appreciation for blondes, brunettes and redheads. Of course, one can't change a Shay to a Heisler with a few buckets of paint.
Japanese engineer Kozo Hiraoka has constructed 3/4 inch live steam models of the Shay, Heisler and the Climax and written separate books on how to build models of each. I'm envious. While I'm up to my ears building a shay at the moment, I've obtained a set of those unique Climax skew bevel gears in 1.6" scale, so who knows what the future holds.