Sometime in the 1950s (?), SP split their timetables from the majority of the Special Instructions until sometime around 1980. The timetables of this era only list the track in use and schedules, you need the Special Instructions to see speed limits and any restrictions - including potential engine assignments. It also lists a maximum load limit of 251,000 pounds for the branch. If I recall, it wasn't long before 251k became 263k, and about 10 years ago was raised again to 286k.

What is of note here is that the speed limits were fairly high, and the maximum car load limit was typical of the largest cars in service. For reasons unknown, SP did not list all the newer road engines (C628, SD35, SD40, SD45, GP20, GP35) suggesting this might have been an oversight, there was no intention of using these classes of engines, or the newer engines were restricted for some reason. It also seems odd that there is not a regular train to Searles to interchange with Trona. In the 1950s and 1960s, how many cars per week did Trona actually interchange? I don't think Trona really boomed until maybe the late 70s as far as rail traffic, but I may be wrong.

I don't think new Special Instructions were necessarily printed each time a new Timetable was issued which would have related in a cost savings. Any changes or corrections would have been done by Bulletin (General Orders today). Finding Special Instructions are hard enough, Bulletin Books are extremely scarce.

There are plenty of errors in the 1966 Special Instructions that would have needed to be corrected. It does state "New Nominal Class" under engine restrictions referring to the 1965 renumbering and change of engine class designations, but under the Exeter Branch it still lists engines under their old DF class, but immediately under that for the Visalia Branch the newer designation scheme is used. Another error is in the tonnage ratings for a GS407 (GE 70 ton #5100-5120) between Wallace to Toyon and MP 139 to Kentucky House of 4000 tons. Obviously this was supposed to be 400 tons, though a GE 70 ton is rated at 4000 tons between Lathrop and Fresno while an SD45 is 12,000 tons. Apparently there was an issue with the AF624 (RSD15) as they are only rated at 5900 tons while the C628, SD35 and SD40 are all rated at 10,000 tons for the same territory.

The GE 70 tons are also consistently shown as having a higher tonnage rating than a EMD SW1 or Alco S3, though the bigger engines would clearly have a higher starting tractive effort advantage.