The Heisler running boards run from the front of the cab to the front of the boiler. They rest on the upper frame bar. The crew can access the running boards from doors in the front of the cab or via stairs from the front sill.
The photos above shows the front of the running boards and the stairs on MRSR91. The stringers on both sets of stairs look like they have been mangled at some point. The next two photos show the running boards and stairs on Cass No 6. The major difference between the running boards for the two locomotives is where the steam supply pipes go through the running boards.
The drawing above shows the left running board. The required dimensions will likely vary from locomotive to locomotive. The 1" hole is for the steam supply pipe. The plate is sawed in two at the hole. A joining plate is attached under the saw cut to facilitate joining the two pieces. The inside edge is cut pack to give clearance for the water supply valve and check valve that is at the same elevation as the walkway. The right side is identical except for cut outs around the compressor.
The two photos above show the installed left side running board. The first photo was taken when the locomotive was nice and clean show how the rear of the running board is attached to the lower part of the cab front using a small angle. The second photo was taken after the locomotive was operated for about 20 hours and not thoroughly cleaned. The steps (described later) are attached to the running board at the top and rest on the front sill on the outside and the frame tray on the inside.
The running board rests on the top of the frame. The outside of the running board is supported by the angle bracket at the back and the aluminum block shown in photo above. The block is a piece of 1/2" square bar stock with the end cut at a 45 degree angle. The block is attached to the cylinder (the area with no steam passages below) with a couple 6-32 SHCS. The angled surface is at the same elevation as the top of the frame. A hole is drilled into the angled surface and tapped for a 4-40 screw to secure the running board.
An angle bracket was located just forward of the end of the boiler jacket. The bracket was made of a ~1/16" thick angle attached to a scrap piece of the smoke box tube. The bracket was attached to the running board with a couple 4-40 screws and the hole through the curved part used to mark the location of a threaded 6-32 hole through the smoke box. This bracket is not used on the right side. Instead, right running board is attached to the mounting for the air compressor.
The drawing above shows the construction of the stairs. All the pieces are cut and formed from 1/16" X 3/4" CFS strips purchased form McMaster. The treads are attached to the stringers with 2-56 hex head screws. The stringers were cut extra long on the bottom and then trimmed to fit after the assembled staircase was clamped to the running board for a test fit.
This last photo shows the parts for the stairs. The bends on th ends of the treads were made by grasping the tread in a vise and pounding the edge over with a hammer.