Capturing the rods in the up or down position shows the wheel hubs as well as the crank-pin offset. Rods-centered covers some or all of that stuff up. In addition, rods-down or rods-up is the middle of a power stroke and an image of that state does a better job conveying motion or power than does one depicting rods-centered. Lastly, builders photos were mostly done with the rods down, again, probably because that's just a better way to display the running gear.
For me personally, I think rods-down is most pleasing to the eye and what I go for. Rods-up is my second choice. I delete most of my frames with rods-centered. To me, it just looks goofy, despite the fact that a working engine is in that position twice as often as either of the other positions. On a slow-moving locomotive, you can usually time the frame to achieve a rods-down shot. On an engine moving at track speed, you need to shoot in burst mode. My camera shoots at 10 fps, which will nearly always get me what I am seeking.
/Kevin
Edited 1 time(s). Last edit at 01/21/2016 07:25PM by KevinM.