The first question should be: Why did St Louis Subdivision have OBS between Southeastern Junction and Marquette, while the rest of the St Louis Sub and the Chaffee Sub operated as a dark railroad?
I don’t know why the 1959 ETT lacks this info, but previous and latter ETT's note that the St Louis Sub between Southeastern Jct and Marquette operated with OBS or ABS (Rule 510a), which is Frisco speak for OBS
Yes, the Frisco employed both APBS and OBS, both of which a variations of an Automatic Block Signal system. There are several discussion on the site; search for OBS or APBS
Nash, MO.-Turrell, AR. CTC was installed in 1954, SE Jct. (Lindenwood)-Nash CTC was begun in 1978, in service to Neely's in March 1979 & completed to Nash June 1979...There was always more traffic on the south end even today, lot of traffic out of Blytheville...Roger
