The John Deere Legacy by Broehl. Page 42 contains the following about the Froelich Tractor.

I believe that he used or copied the Rumely steam engine transmission because Rumely was the only steam engine manufacturer to use a reverse gear; everyone else reversed the engine to back up.

Just how the power was delivered to the rear wheels is not clear; however, a friction drive like that commonly used to drive thresher cylinders was utilized. This consisted of a compressed friction wheel on the powershaft of the engine that engaged a plain iron pulley on a driven shaft. The tractor mechanism differed from the thresher mechanism in that a spring held the mechanism out of engagement, and to start the machine as a tractor, the operator at the steering wheel rode with one foot on the end of a long lever that forced the friction wheels together.

This passage caused me considerable headache while researching the Froelich tractor, as there is very little written about the Marsh valve gear the Rumley steam traction engines used—as documented in my other post, this forum, seen in this video, and subject to this patent (and this one). It took a while to realize that Broehl should not be considered a reliable source when it comes to the Froelich tractor. The Rumely steam traction engines of the time did not use a reversing gear train but the use of the Marsh valve gear also reversed the operation of the engine just like other steam traction engines. It is also the only source I have found to claim that the main beams were laminated to “bend together at the front over the front axle, while at the rear they were spread out practically…parallel to each other” (42). None of the replicas nor period sources seem to show the main beams bending.

Reversing a steam engine requires reversing the operation of the piston. Unlike with an internal combustion engine, the piston is powered on both the “upstroke” and “downstroke.” On the upstroke steam is injected behind the piston head and once it reaches top dead center, steam is injected in front of it, driving it back to bottom dead center. Thus to reverse the piston, it’s stopped wherever it is in the cycle and steam injected on the side opposite where it was last injected. Injection is controlled by the “valve gear.” Below is an animation of the Walschaert valve gear common on steam locomotives, in both forward and reverse.

Below is a very rough animation of how the Marsh valve gear did the same. The system was actually relatively simple: the valve gear’s crank rod was tied into the motion of the piston via a drivetrain, which drove two spur gears with the valve gear crank rod attached to the latter. Both spur gears were mounted on a lever that could rotate around the axis of the drive train they were attached to. By rotating the lever so that the end of the crank rod was either above or below a horizontal plane extending from the center of the drivetrain axle, the valve gear crank rod’s motion would change, thus reversing the valve gear.