While Timetable and Train Order operation isn't quite dead yet, the "timetable" part of it almost certainly is, even on the one line that officially still uses the system. This is because for all the talk of superior and inferior trains the LIRR does a pretty good job at making sure trains know explicitly when they have to take sidings due to the Manual Block system that covers the more traditional T&TO. Of course that is what I thought until I started going some research on METRA's Blue Island interlocking tower located on their Rock Island division.
Anyone familiarizing themselves with the METRA commuter network may have noticed the strange lack of any automatic signaling system on the Beverly Branch portion of the Rock Island District. The Beverly Branch is an interurban themed loop off the Rock Island main line that sees the majority of Rock Island district trains save for a few rush hour express runs. Running 6.5 miles from Gresham Junction to Blue Island – Vermont Street about 5 miles of that is not covered by any sort of automatic signaling, which under GCOR means ABS or CTC rules. Of course what the line lacks in signals it makes up for in stations with a total of 10 filling the unsignaled section between the new CP EZLIABETH and the 125th St interlocking.
* A proceed indication displayed by the controlled signals governing entrance to the non-signaled territory or verbal authority from the train dispatcher or control operator will authorize trains which have scheduled passenger stops, as indicated in the Employee Train Schedule, to enter and run with the current of traffic in the non-signaled territory. A train must not follow another train until 10 minutes after the preceding train has departed.
* Road Dispatcher at Gresham Junction and Control Operator at Blue Island must hold trains at the controlled signals governing entrance to the non-signaled territory of the Beverly Sub District until 10 minutes after the preceding train has passed. They must also notify any train which is followed by a train scheduled to precede it.
* Trains not indicated in the Employee Train Schedule as operating via the Beverly Sub District and trains moving against the current of traffic will be authorized by track permit after an absolute block has been established in advance of the movement.
What we have here is a somewhat informal version of timetable operation which keeps the time separation components, but discards many of the issues about actually keeping to your assigned schedule. Only scheduled trains may enter the time separation area on signal indication, but it doesn't matter what that schedule is or if they are following it. The rationale behind this the high density of stations which all trains on the line must stop at. With stations less than one half mile apart each there isn’t much room for trains to build up a dangerous amount of speed and it is not difficult to observe any traffic running on the line ahead.
Even with the frequent station stops and a 30 mph maximum line speed, a second layer of safety is provided by the operators (now dispatcher in the case of Gresham) who cannot display a signal into the time separation without waiting 10 minutes since any preceding train was admitted. This time separation requirement applies for trains within the block which cannot depart a station any time sooner than 10 minutes after the preceding train has departed. The signals at both entrances to the section can either display some version of Clear and Restricting, so just like in manual block systems the Clear served as a manual block proceed indication that is valid all the way to the next interlocking, unless the train is held up by the at station 10 minute rule. At the new CP-ELIZEBETH (replacing a portion of the old Gresham Junction interlocking) you can see the proper direction track westbound signal can display G/R on the upper head and R/L on the lower. The reverse track can display G/R/L on the lower head.
While in theory time separation could be employed equally well on a bi-directional scheme with admittance to the blocks governed by the operators/dispatchers at either end, in practice all reverse direction moves must get a track warrant. Moreover all non-scheduled trains must also get a track warrant as no scheduled train skips any stops and the station stops serve as safety critical time checks.
Unfortunately all these stops come at the cost of timeliness with the 5 miles in the un-signaled territory taking upwards of 20 minutes to traverse according to the schedule. That means that the line would not do much worse if it was simply converted to Yard Limits and trains run at restricted speed (although you might not be able to make any stops). In terms of train frequency the shortest headways on this line are 13-14 minutes apart, although to handle the peak traffic other trains can be sent via the Main Line with its full CTC protection.
In advance of each exit interlocking, CP-ELIZABETH to the east and 125th St to the west, there is an automatic distant signal allowing the block in advance of exit interlocking to be run under ABS rules. The setup is reminiscent of Amtrak style Rule 562 operation with cab signals with fixed waysides only at distant.
The history behind why this stretch of line was left signaled the way it was is a bit of a mystery, especially considering that the Rock Island was the first to make use of RTA funds to improve its service. One probably culprit are the large number of grade crossings in the Beverly Hills section of the line with 24 being located in the unsignaled section. Automatic crossing protection using DC track circuits would have required complex logic when combined with automatic block signaling. Demands from the time to improve crossing protection or an effort by the Rock Island to avoid paying crossing tenders may have lead to the discontinuance of automatic signaling in that area. The practice may have also evolved from older manual block or timetable operation that was never upgraded due to the slow speeds and frequent station stops.
How much longer this practice will continue is a matter for speculation. The Rock Island district is already targeted for a pilot PTC system and such dark territory is the thing that PTC generally eliminates as part of required signaling upgrades. However PTC could be used as an overlay to provide actual enforcement of the current time separation and remove any necessity for track circuits on this stretch of track. The time separation rules would of course probably be left in place for when the PTC system inevitably doesn't work.