On the SP:

H = Hotbox detector found a hotbox, proceed at reuduce speed to [the singal case with the] readout.

M = continue on Main track [to pick up train orders granting movement authority].

P = "Proceed" -- there was also one of these EB a couple of miles west of Antelope.

S = take Siding -- there is still one on each controlled signal to enter Castroville (or the Ord branch). Even though still listed in the current timetable (if the S is illuminated you can pass the signal displaying STOP to enter the siding), apparently very few crews and the DSs know what they mean anymore.

W = Wait [here] -- there are still two of these on the signals on #1 and #2 tracks (one for each signal) WB at Stanford [flat].


There were others, but I can't remember them off the top of my head.