The current vision of a reverse trike layout is not the first.
Steam Scooter (Standing) - Think electric scooter. Not enough space for components.
Steam Scooter (Sitting) - e.g. Vespa. Still no good arrangement of components. Instead of a cylinder- or slide-valved engine, engine design had the cylinder with the crank to expose and cover ports in a valve plate.
Steam Motorcycle w/Sidecar - Driven wheel was to be the sidecar wheel, complete with crank drive. Too asymmetric, no place for 2nd cylinder.
Reverse Trike - Single wheel ideal for driven wheel, and that's the common setup with reverse trike bulids. More stable than a forward trike.
On a full-size locomotive, the pistons are rigidly fixed to the locomotive frame, but the wheels driven by the pistons are unsprung on a suspension. The nature of the crank mechanism means that some of the force of the piston is trying to push the wheel up or down in its suspension as the wheel rotates, but on a full-size locomotive, that force is a tiny fraction of the weight on the driver and can be neglected.
On our steam trike, that unwanted force from the piston-crank mechanism becomes significant. To avoid the issue, the cylinders will be fixed to the same structure as the driven axle and this structure will form a swing arm for the driven wheel. By placing the hinge point at or around where the cylinders are, the cylinder mass will be mostly sprung.
In their 1982 SAE paper Three Wheeled Vehicle Dynamics, Huston, Graves, and Johnson conclude that the COG of a three wheeled vehicle with two front wheels should be located one-third of the wheelbase back from the front axle
The commercially produced Polaris Slingshot appears to have a COG about a third of the way back from the front axle according to customers' weight measurements
Driver/Passenger - Assume 200 lb each
Accumulator - Depends on the design capacity. 50-60 lb for a 15 gallon accumulator.
Burner - 30-40 lb
Steam generator coil - TBD
Water/Fuel - TBD, but water and fuel represent a variable weight that should not affect the stability of the vehicle as they are used and should thus be placed around the center of gravity. Luckily, this is about where such tanks are on prototypical tank locomotives.
Driven wheel - Depending on the tire size, this could be up to 100 pounds
Engine - TBD; the large steel/iron steam cylinders, connecting rods, and valve gear will contribute to the overall weight
Battery - A group 27 flooded cell lead acid battery weights about 50 lb