Not sure if this is helpful or not Pat, I'll let you decide. This topic is back from 2017 and our resident sage of mostly all things steam Hot Water answered twice to the original poster's question. The link is to his second reply, which states that the engineer(or fireman in the case of SP) had to turn off the headlight to turn on the reverse or back lights. Please look at this to confirm that I read that right.
Most of the replies say the same as Hot Water, in that it is switched on to off for the headlight, and backups switched on. There is also some other bits talked about with switchers as well as an engine yielding to another. Have a look and see if you think that it makes sense. Another reply cited New York Central rule from I think 1956 which is above Hot Water's reply.
