Cab signals and headways on the Red and Orange Lines

I was going to post this in the 'MBTA to remove seats from Red Line cars' thread, but I decided this may be worth it's own discussion.

What is the theoretical headway between trains that is allowed by the cab signal system on the Red and Orange Lines? Does the system vary at all based on location? I'm looking for system limitations and am not interested at the moment in real-life operational constraints such as trains that break down, dwell too long at station platforms, etc.

I recall reading a newspaper article years ago which stated that the minimum headways that are possible on the Red Line are 3 minutes. Of course, we all know how accurate newspapers are when it comes to trains, so take that with a grain of salt.

I've seen it closer than three minutes.....I've been at Park St...seen one pull out and another pull in right behind it.

Orange Line is as low as 1 minute. While waiting at Back Bay, a Forest Hills bound train came in, did it's thing and pulled out. Before it was even fully out of the station, that silly little announcement came on and said "the next Orange Line train to Forest Hills is now approaching." 30 seconds later we got hte announcement saying "the next Orange Line train to Forest Hills is now arriving." 30 more seconds and it was in the station.

That is possible - if everything works as designed. Add one slow order to the mix and the time gets longer, and the lowest throughput controls the headway. The Orange Line has no tail tracks in service, where an arriving train at a terminal is slowed by a reversed switch, at both terminals, the trains change ends on the platform, and can run through the approaching facing switches at 25mph. The Red Line has this condition at Ashmont plus the 10mph curve at Harvard. Those are what limits the scheduled headway. The curves at Harrison Sq., Andrew and South Station are also signalled for 25mph but designed for 35mph (which is what they were signalled for with the old wayside system).

Red, Orange Line signal upgrades eyed by early 2022

MBTA overseers gave a green light Monday to a $218 million contract for work to upgrade the train control signal systems on the Red Line and Orange Line, the final piece of work that T officials say is critical to meeting the system's service reliability goals.

https://www.wcvb.com/article/red-orange ... 2/23555825" onclick="window.open(this.href);return false;

Reverting back to wayside and trips like the Blue Line has would be a much more cost effective way to meet the system's service reliability goals.

(copied from another post)
Blocks must be long enough and signals/trips set so that if a train overruns "a red signal" and is tripped, it will stop before reaching a train up ahead. The present Green Line subway does not satisfy this. If trips were installed, all other things unchanged, and if a train got tripped, the next train ahead could be just ten feet away.

Meanwhile, if, say, a delayed soutbound Red Line train is loading at Park St. and trains following are directed to hold up, open doored, at Charles, Kendall, Central, etc. it will take longer to get back on schedule compared with the first train behind at Charles, the next one (closed door of course) on the Longfellow Bridge, the next train at Kendall, the next train halfway between Kendall and Central, etc. I am not sure whether the block structure will permit this.

The article doesn't say what technology would be used, but one hopes a more modern system that does away with fixed blocks such as CBTC would allow a higher frequency plus have other advantages such as more accurate countdown clocks and eventually true ATO.

Jon