The conventional "rod" locomotive gets it's nick name from the side
"rod" or metal bar that connects all of the drivers (driving wheels) to the
steam cylinder. The drivers were fixed one in front of the other and could not
rotate like the drivers sets or "trucks" on the the geared locomotives.
This limited the sharpness of curves the rod locomotive could travel through on the
railroad. The more drivers a rod locomotive had, the less it could tolerate
the sharper curves on the railroad. The rod kept the drivers on one side moving in
unison. Note the size of the drivers as compared to those on the geared locomotives.
Their size (sometimes taller than an adult person) was great for speed, but
reduced the load carrying power of the engine.