Discussion in 'Central Division Employee Timetables (ETTs)' started by john, Mar 13, 2011.
FRISCO Central Division ETT No 37, April 30, 1950
Does anyone know what an overlap block system is? (As indicated on page 3, item 7 [Block Signals]) I know what an overlap is, but not OBS. I am guessing that once you enter, it over laps all the block in that system. It protects the Arkansas River bridge and Jenson tunnel and look like they are short blocks. Very interesting.
From 'Railway Track and Structures Cyclopedia' Published by American Railway Engineering Association, 1921. As found on Google.
>>>OVERLAP An arrangement of track circuits for block signals originally introduced as a substitute for distant signals. With a block section extending from A to B a track circuit for say 2,000 ft beyond B is arranged so that when this space is occupied by a train or car the signal at A will be held in the stop position the same as though the train were between A and B. Thus when signal A is cleared an enginem accepting this signal knows no train is in the track circuit in advance of Signal B. Where a line is fully equipped with distant signals so that enginemen will never need to slacken speed to make sure of the indication of the home signal before passing it the overlap is generally deemed unnecessary. In one scheme of single track automatic block signaling overlaps are used to prevent a collision between trains moving toward each other which without the overlaps might pass clear signals at the same moment. The opposing signals are so situated that each train will encounter a stop signal before it can meet the other in a butting collision<<<
I noticed in the timetables posted by John there are several references to OBS. The oldest rule book I have is dated 1957 and there is no reference to OBS. Sometime between 1948 and 1957 the ABS signal system must have been upgraded (?).
Also, I was not aware Frisco used staffs (signaling) into the 40's
Thanks for posting the timetables.
As I find the time I'll try and post a few more of the ETT's that I've collected, attempting to fill some of the gaps in those posted online. Working with a few of these "oversized" timetables makes you appreciate all the work that Karl and the other posters have done.
I believe that another affectation of OBS was that the Frisco did not use A-block signals. In some "newer" ETT's, OBS and APBS are not distinguished, except that the Rule 510(a) is applied to the OBS segment. The Creek Sub in the System ETT Number 1, Oct 17, 1971 is one example. I have 40's rule book that I will check.
Rule 510a applied to both Creek and Oklahoma subs (and I am sure others). Time Table No 36, Southwestern Division, May 15, 1948 show Creek and Oklahoma Sub as OBS. Rule 510a must have been the change that did away with OBS (?).
Rule 510(a) applied on the Creek Sub to the south end of Henryetta at which point we had an ABS "A" signal. If that signal was red, we would pull past the signal and wait five minutes. Because it was not a CTC signal we didn't call the dispatcher because he couldn't tell us whether there was an opposing train in the block or not. ABS extended from south Henryetta to Sherman if my memory serves me correctly. Terry
At least through 1952, the Frisco still made a distinction between APBS and OBS in the ETT's. A May 1957 ETT(which post-dates the 1957 Book of Rules) refers to "everything" as ABS but adds the note "Rule510(a)" to those segments that were previous noted as OBS. It is my understanding that these OBS segments remained OBS until they were deactived or replaced by CTC. The Frisco simplified the ABS Rules in 1957, but needed to keep Rule 522 from the 1946 book as Rule 510(a) in the 1957 version. The 1957 rule was modified to include the verbage: "Within defined limits so specified on time table or by general order, the following will govern:"
It is also my understanding that APBS was slightly better than OBS with providing protection against opposing movements, and hence Rule 510(a)
Attached are the appropriate rules from the Mar 1, 1946 Book of Rules:
504-520 ABS Rules
522-523 OBS Rules
Thanks for the upload! Interesting reading!
Now I know the history behind rule 510(a). Thanks Karl!
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