Thanks largely to research by Yury Kats, I'm confident now that the post marked R is a ring post, signalling the driver to ring his bell, and not a resume speed marker. The first strong evidence is the following paragraph from page 229 of Elements of Railroad Track and Construction by Winter Lincoln Wilson (John Wiley and Sons, 1915).
Whistle and Ring posts are described as identical except for the letter displayed. Sure enough, our mystery post has just that relation to the whistle posts near the Bedford end of the trail. Past the Bedford end Yury saw matching W and R posts on the Reformatory Branch Trail and found them described in the trail guide as whistle and ring posts, respectively.
Last week: 0 miles due to a terrible head cold. This week doesn't look good either, with high winds forecast tomorrow and rain on Tuesday.