Summary: The process of aligning the driver's crankpins
Steam locomotive drivers are quartered. The crank pins are set 90 degrees apart. Without quartering, it would be difficult to start the locomotive.
Many railroads had the right side crank pins leading the left. When the right rods were at their lowest point (the "bottom quarter") the left side was on front dead center. The Pennsylvania Railroad was the largest user of left-hand lead locomotives. The theory was because the PRR had so much multiple-track territory; the most solid part of a multiple-track roadbed is toward the center, and since the side of a locomotive that has the lead is the side that pounds the track hardest, PRR wanted the locomotive to pound the most solid part of the roadbed - the left hand side.
The process of aligning the pins is called quartering. This is done in a purpose built jig. All the drivers must be quartered in the same manner to avoid binding, caused by one driver not being in phase with the others. All must be done at the same time, using the same jig, as no two jigs are alike.
When working on a steam locomotive, it is very important, if the drive train binds or jams, the decoder could be damaged or destroyed. Older brass may need this checked and adjusted to obtain smooth operation before conversion to DCC. Quartering is a big job, requiring a lot of time and patience. The entire drivetrain must be adjusted and any corrections made so that the wheels will rotate freely. When re-assembling a steam locomotive mechanism it should roll freely in forward and reverse with light pressure pushing it (before the gearbox is assembled). Any jamming or binding of the motion will need correction before installing the gears and completing the project.
As a rule, if the engine runs fine, do not mess with it. The process of aligning, adjusting and quartering should only be done if there are problems. Tolerances are in the range of a few thousands of an inch.
In real life, a locomotive could not start if the drivers were not quartered. They also spent a lot of time adjusting and shimming bearing blocks to line up the wheels correctly with respect to the rest of the rotating assembly.
NWSL makes a number of specialized tools for maintaining locomotives, such as pullers for removing wheels from the axle. They also sell gearboxes, motors and wheel sets. For more info and tools, see the NorthWest Shortline Website