David's photo below caught
UP's Big Boy #4014 with the Walschart Valve Gear in its neutral position because it was parked in West Chicago, IL. The vertical link with the big slot is the Expansion Link. The bar going to the right from that link is the Radius Bar. The engineer controls the position of the radius bar in the expansion link slot with the Johnson bar. In the early steam locomotives, the Johnson bar was a big lever connected directly to a link that raises and lowers the radius bar. In big locomotives, the Johnson bar controls a motor that turns the end of the link on top of Dave's photo to raise the radius bar up or down in the slot. When the end of the radius slot is below the center of the slot, the locomotive goes forward. As you would expect, when it is above center, the locomotive goes backwards. This is illustrated by the following two screenshots.
I caught it in the forward position while it went slowly around the curve on the east side of Elmhurst, IL. The radius bar is a little below the center, or pivot point, of the expansion link.
lakeshoremodelrr caught it slowly backing into its display position in West Chicago, IL. The radius bar is a little above the center of the expansion link.
WGN TV chased it with a helicopter. The good news is that it caught the locomotive running pretty fast. The bad news is that during most of the video, the running gear was blocked by trees or buildings. But I found a part that has a clear shot of the running gear. Unfortunately, the lighting is not very good.
Fortunately, UP didn't paint the rods black so we can see the white expansion link. I used GIMP to crank up the contrast +20. The radius bar appears to be lower in the slot when running at higher speed.
I know the position of the radius bar in the slot controls the valve's cutoff. When starting the locomotive, you want the valve open during most of the piston's travel to apply full boiler pressure to the piston most of the time. But when running at speed, you want to cut off the steam supply soon after the piston travels in the opposite direction so that the rest of the travel allows the steam to expand and use more of the thermal energy that was put into the steam by the superheater. This reduces the fuel consumption when running at high speeds. I read that, when operating at speed, the throttle would be wide open and the speed would be determined by the amount of cutoff.
There are more screenshots of the valve gear position in my Big Boy notes.
UP's "donation train" to the Railroading Heritage of Midwest has two steam locomotives being pulled by diesels. So we have an opportunity to watch the valve motion (more accurately, lack of motion) when it is in the neutral position but the locomotive is moving.
This appears to be a different valve gear design.
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