throttle located on the left side of the boiler just above the steering gearbox and blow down valves

Steam generated in the boiler is controlled to the engine through the throttle.  A steam line at the top center of the boiler conducts steam from the boiler to the end of the throttle.  Inside the throttle a moving assembly regulates the amount of steam allowed to pass through the throttle.  Steam leaving the throttle, through a port in the side of the throttle body is piped to the superheater and then to the engine cylinders.  The Stanley throttle is known as a "balanced throttle" because steam pressures are equalized within the throttle assembly and thus the pressure of the steam can not change the throttle setting once opened.  The operation of the throttle is controlled by a lever arm just below the right side of the steering wheel. 

HOW THE THROTTLE CONTROLS THE SPEED OF THE CAR

The major parts of the throttle are the throttle body (338), and the throttle stem (339-A, B, C).  When closed, a valve disk (339-B) is seated against the throttle body (338).  As you move the throttle lever on the steering column to open the throttle a mechanical linkage causes the throttle stem assembly to move (towards the left in the drawing).  As that happens the valve disk (339-B) unseats from the throttle body (338).  Once unseated steam is admitted to the inside of the throttle cylinder (339-C).  This cylinder has a hole in the side of it that aligns with the port in the side of the throttle body (338). 

Even though the valve disk (339-B) is admitting steam to the throttle, it can not yet leave the throttle because the hole in the side of the cylinder (339-C) is not yet in align with the opening in the side of the throttle body (338).  This action allows an equal amount of steam pressure to be on both sides of the valve disk (339-B) and thus with no pressure differential on the disk it stays were it is positioned.  This is why the throttle is called "balanced".  It won't move on its own once steam starts flowing past the valve disk.  When the throttle is fully closed there is still a little bit of leakage past the seat of the valve disk and the throttle body but that steam is trapped inside the cylinder (since the hole in the cylinder is not aligned with the hole in the throttle body) and can't escape and thus the pressures on either side of the throttle disk are still equal.

As the throttle is opened more, the hole in the cylinder (339-C) starts to align with the hole in the side of the throttle body (338).  As this movement occurs a very small segment of the hole in the throttle cylinder (339-C) overlaps the hole in the throttle body (338).  Steam can then escape from the inside of the throttle cylinder (339-C) through the hole and out the port on the side of the throttle body (338).  As motion of the throttle continues the opening in the throttle cylinder (339-C) and the throttle body (338) becomes bigger and more steam is allowed to pass.  Maximum steam flow occurs when the hole in the throttle cylinder (339-C) is fully aligned with the port in the side of the throttle body (338).

The other parts of the throttle include a clevis (341) for connection of the throttle stem (339-A, B, C) to the mechanical linkage of the steering column lever.  There is also graphite packing, a packing gland (343) and a packing nut (344) that keeps steam from leaking from around the throttle stem (339-A, B, C) as it slides in and out of the throttle body (338).