Tramming the frame is the process of measuring the frame and aligning the various components of the running gear, so everything runs straight and true.  At the end the process the crew will have the dimensions to machine the shoes and wedges, so the drivers are centered in the frame, perpendicular to the centerline, parallel to each other and at the proper spacing.  Tramming also presents the opportunity to confirm that the frame itself is straight and that the Bssell post, lead truck pivot, drawbar pin and buffer are centered between the rails.  
The process starts by establishing a reference line running the entire length of the frame through either the cylinders or down the centerline of the frame.  The oldest methods use steel piano wire that is held under tension and becomes the foundation of all the work that follows so accuracy is extremely important as well as a strong setup that will not deflect for the duration of the work.  Then using straight edges, squares and “trammel points” the individual dimensions are measured.
The final stages of the manual process are determined by what dimensions are needed, in 557’s case the shoes and wedges are in excellent condition and dimensions for centering the driving boxes in the frame and the hub liners are needed to complete the project.
With 557 we are using the relatively “new” technique of Laser Tracking (compared to the wire method that originated in the 1890’s).  The laser tracking unit measures the surfaces of the entire frame and creates a three-dimensional point cloud that is then manipulated in CADD  (computer-aided design and drafting) software to create measurable surfaces that will generate the desired numbers for the machinist to finish the parts.
At the end of the day the accuracy of both processes should comparable and yield the same results.  The advantages of the laser tracker is the data can be collected in one day verses the multiple days required for the traditional method where temperature changes, faulty setups or clumsy bystanders can cause errors that can render a part or all of the measurements invalid.