When Chicago police officers are on Metra’s right-of-way, whether responding to a call about a suicidal subject or an accident or pursuing criminal suspects, there’s a practice that’s generally followed, according to the commuter rail agency.

“The protocol is for CPD” — the Chicago Police Department — “to contact their 911 center, which contacts our police dispatchers, who then contact our train dispatchers,” Metra spokesman Michael Gillis said Tuesday.

Trains are then told to halt because people may be on or near the tracks.

But before Chicago Police Officers Conrad Gary and Eduardo Marmolejo were struck and killed by a South Shore Line train on the Far South Side on Monday night, Metra was not asked to slow or stop trains because of police activity, Gillis said.

The tracks at that point are shared by South Shore and Metra Electric District trains, which are electric-powered, drawing from the same overhead lines. Metra handles train dispatching for South Shore — a separate commuter rail agency — in that area.

“We were not contacted . . . we were not asked to stop trains before” the tragedy occurred, Gillis said.

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Michael Noland, South Shore’s president, added, “I’m not aware of our engineer being notified.”

Gillis didn’t know how often trains have been stopped in this manner in the past, but said, “it’s not uncommon.” What’s more, such calls don’t just involve “CPD but other municipalities,” too, since Metra cuts through more than 100 suburbs on 11 routes. The South Shore runs between Chicago and South Bend, Ind.

Chicago police officials said they weren’t immediately able to provide details. The Monday night incident remains under investigation.

“Detectives are reviewing any and all available video . . . all CPD radio transmissions, police vehicle GPS records, and of course conducting a formal interrogation of the offender,” said police spokesman Anthony Guglielmi. Police were questioning a man who they suspect fired the shots that prompted the officers to be on the scene.

The officers were responding to a report of gunfire around 6 p.m. at 101st Street and Dauphin Avenue by the railroad tracks. They spotted the suspect at some point, and were searching for him on the tracks — which are elevated above street level — when the accident occurred.

As the officers walked south on the tracks, about 10 yards apart from each other, there was a Metra train heading north, officials said.

It’d just left the 103rd Street station so likely wasn’t going very fast, Gillis said.

The officers apparently saw that train, but police believe its noise may have “drowned out” the sound of a South Shore train traveling much faster — perhaps as much as 65 mph — as it headed south toward a switchover to a different set of tracks near 115th Street.

That southbound train struck the officers from behind, killing them instantly, officials said.

It’s unclear whether that train engineer saw the officers, blew the horn or applied the brakes before impact and, if so, when.

The sun had been set for about two hours, and the train lights may have been dimmed, as is practice when two trains are about to pass each other, just as two automobiles might do with their brights.

Rail right-of-ways in general are dangerous places, more so than many people probably realize, according to sources in the railroad industry.

In the dark, trains might seem distant when they’re not, or based on its lights, a train in the dark may look “like it’s not even moving,” one railroad veteran said. Trains also move more quickly than people may think.

South Shore and Metra Electric District trains, because they’re electric-fueled, run relatively quietly.

Along this busy stretch, which may have been unfamiliar terrain for the officers, there are at least four sets of commuter tracks, and about 150 commuter trains pass through on a given weekday. There are also several freight tracks nearby that carry freight traffic and Amtrak.

The railroad veteran, who asked not to be identified, suggested police consider calling off future foot pursuits when they involve rail property, much like high-speed car chases are often stopped to avoid accidents.

“It’s not worth an officer’s life,” said the railroader.

The lights from this kind of train probably illuminate from the cab a “couple hundred feet,” and depending on the weight and speed of the train, it could take three-quarters of a mile for the train to come to a complete halt when the brakes are engaged, the industry veteran said.

Details on speed and braking are likely captured on the train’s event recorder — standard equipment in trains and planes. The South Shore train doesn’t have video, but the Metra train does, though Gillis couldn’t say if any usable images were captured.

The National Transportation Safety Board, which investigates many serious transit accidents, is “monitoring” the situation, a spokesman said. The Federal Railroad Administration, which regulates U.S. railroads, has been in contact with South Shore and Metra, officials said.