If you missed it, the MBTA unveiled countdown clocks for the Red Line at South Station last week to tell customers down to the minute when the next trains will arrive. The pilot program could expand by year’s end to the entire Red, Blue, and Orange Line systems, ending the uncertainty associated with waiting at stations since the dawn of the subway.

Amid their LED glow, I wondered if the countdown clocks might have historical precedent — some analog dial from the early 20th century, perhaps with signals conveyed by vacuum tubes or radio waves.

Not quite, said Bradley H. Clarke, transit historian and president of the Boston Street Railway Association. But they are part of a long tradition of trying to bring passengers out of the dark about their next train, at least a little bit.

When Boston opened North America’s first subway Sept. 1, 1897, it was not a complete subway system but a tunnel that allowed a web of streetcar lines running to and from different neighborhoods and suburbs to duck underground and avoid the morass of Tremont Street.

That made for chaos at Park Street Station, where there was no rhyme or reason to which berth a particular trolley would stop at each time — until Jan. 30, 1899, with the installation of a massive overhead sign listing all routes with a set of light-up numbers next to them. That gave customers a brief heads-up about where their cars would stop, making the platforms that much saner.

As the lines using Park Street dwindled, the remaining trolleys were assigned fixed berths — as is the case today with the Green Line’s B, C, D, and E branches — and the sign was retired May 12, 1934, Clarke said.

The next major innovation was the amplified public address system, which only feels like it’s been part of the subway forever. It debuted on the Boston Elevated — precursor to the MTA and today’s MBTA — June 21, 1946, allowing customers at Park Street, Scollay (today’s Government Center), and Washington (Downtown Crossing) to know en masse if trains were delayed or if riders should wait for a crowded car to pass because another was close. The T did not install a centrally controlled announcement system with speakers in all major stations until the 1970s, Clarke said.

A host of modest innovations have followed, including lights above ground at Symphony and Prudential to tell customers if they should hurry because a train is near; lights in the lobby at JFK/UMass to end confusion over whether the next inbound train is coming on the Ashmont- or Braintree-branch platform; a map with light bulbs at Maverick Station that draws on track signals to show the general location of all trains on the Blue Line. More recently, the T has experimented with flat screen, computerized versions of the same at a few stations.

And then there is the method in-the-know riders have long used to tell when a train is a minute or so away: peering into the tunnel toward what are called track-walker lights. They switch off when a train is approaching from behind, providing an alert to crews working on the tracks.

But those lights are only perceptible from the platform at some stations, unlike the telltale breeze that Clarke called “the best indicator of all,” the whoosh trains have pushed ahead of them since they first went underground. “And that is as low-tech as you get.”

Rumble on the Tobin Bridge

The news about a light fixture falling on the Tobin Bridge prompted reader Daryl Smith of Charlestown to ask about an unsettling rumbling from the bridge that has been shaking homes on Mount Vernon, Prospect, and Chestnut streets, near the Boston-bound side of the bridge as it approaches the tunnel below City Square.

“It is especially noticeable in the middle of the night when empty dumpster trucks bounce over it. It sounds like thunder,” wrote Smith, speculating that the culprit might be a misaligned expansion joint.

Turns out the problem is a trench drain on the bridge’s down ramp — intended to funnel water away from the tunnel — that has warped, causing its covering grate to rattle. The state discovered the problem recently while drawing up plans for a 1,000-foot milling and repaving job for the tunnel and the beginning of the bridge, Department of Transportation spokesman Michael Verseckes said.

The state does not have a cost estimate yet, but the work should be completed this fall. The trench drain will be removed and not replaced — existing catch basins should be sufficient for tunnel drainage — which should end the clatter, Verseckes said. And in case the noise is coming from the expansion joints, they will also be inspected as part of the work, he said.

Heat nothing new for the T

How hot is it on the T? Not any hotter than recent years, at least according to the MBTA’s own metrics. Anecdotal reports of air-conditioning failures on the rapid-transit system prompted me to ask T spokesman Joe Pesaturo whether the systems were more balky than usual — or if, perhaps, the T was easing up on the AC to save money.

Not so, Pesaturo said. Customers logged 57 complaints about faulty air conditioning on the subway from June 1 to Aug. 15, and 143 AC complaints on all modes. That was fewer overall than in the same period last year, when 54 subway and 179 total air-conditioning complaints were logged, or the year before (71 subway complaints, 148 total).

But not everyone who rides in a hot car takes the time to register a formal complaint. The T’s own maintenance staff recorded 117 cases of AC failure on the Red Line alone from May 1 to Aug. 15. If that sounds like a lot, consider that Red Line trains made about 30,500 trips in that time. And the T recorded more failures (131) in the same period in 2010, though not last year (95).

Work on 128 causing a shift

Sue in Woburn e-mailed to ask about dangerous conditions along Route 128 in Burlington and Woburn at several exits and on-ramps. Restriping has shifted traffic and eliminated what used to be comfortable merges, while all the lanes are now separated by solid instead of dashed lines. That makes it harder to merge, and less likely that right-lane traffic will move over to avoid cars from the ramps.

“[Now] you must either stop cold or fling your car out there and hope the car in the first lane can slow in time,” she wrote, asking if warning signs could be posted for drivers on the highway as well as the ramps, for the duration of what appears to be temporary work. “Further, there is no appearance of any activity for a few weeks now. How much longer will the public’s safety be endangered? Finally, if I misinterpret and this is a permanent condition, it is a danger to public safety that must be corrected.”

The lane shifts and striping were put in place to accommodate an $11 million milling and repaving job for two miles of 128 (also known as Interstate 95), work that includes repairing highway bridge decks, installing a new drainage system, and reconstructing the median. Contractor Aggregate Industries won the job last year, and the project should be finished in fall 2013, said Verseckes, the state spokesman.

To preserve four lanes in each direction around the work sites, the state closed the left lane on each side of the highway but absorbed the breakdown lanes as travel lanes, painting solid lines to reduce unnecessary lane changes while traffic is shunted around the construction. That also meant eliminating the acceleration, or merge, lanes from the on-ramps.

Verseckes said the state has put up some signs on the ramps to remind merging drivers to yield and some on the highway to indicate that the breakdown lane is in use. But “I don’t think we want to plaster the area with signs, because that may cause more confusion, and we certainly think people can make that adjustment temporarily to the new configuration,” he said, asking for patience.

As with the ongoing Massachusetts Turnpike work in Boston’s Back Bay, the Transportation Department has been fielding questions from drivers wondering why they rarely see workers; both projects are being conducted at night, to avoid interfering with heavy daytime traffic volumes, Verseckes said.