In the early 1870s, an émigré painter watched from a railway footbridge as a steam engine left a station on London’s suburban fringe. His name was Camille Pissarro and he was developing a style of plein-air painting that would soon be called “Impressionism.”

Pissarro and a fellow émigré, Claude Monet, only stayed in London for a few months. By April 1874 they were among the painters holding the first Impressionist exhibition in Paris, the subject of a retrospective that runs until July 14 at the Musée d’Orsay and opens on Sept. 8 at the National Gallery of Art in Washington D.C.

But London was one of their early muses. Monet painted the River Thames and the Palace of Westminster, among other central landmarks, while Pissarro captured scenes in suburbs where houses and train tracks were replacing forests and farmland.

I have a special interest in Pissarro’s train painting because it shows the neighborhood where my wife grew up — in a Victorian home rendered as a “smudge” on the Impressionist’s canvas, as my father-in-law says.

The railway, shuttered in the 1950s, is now a nature path where our kids forage for blackberries during visits to their grandparents.

On our last visit, I decided to find out what Pissarro saw in that train, and what his early London paintings tell us about Britain’s Victorian past. I learned that his brushstrokes captured a moment of dramatic urban transformation whose impacts on the city’s layout are still visible today.

My Pissarro project entailed long winter walks, trips to museums, a ride on a vintage locomotive and a dash of investigative reporting around an arcane mystery. My main guide was my father-in-law, a former “trainspotter” with a white-hot interest in railroad history.

‘Fog, snow and springtime’

A 1990 history of my in-laws’ area describes the old railway as “lost.” But, like other locations that Pissarro painted in Southeast London, the site where the tracks once ran wasn’t hard to find. I could see it through a bedroom window, just beyond the camellia and winter jasmine.

Pissarro, a Danish citizen fleeing a Paris suburb during the Franco-Prussian War, was used to being an outsider. He had been born on the Caribbean island of St. Thomas to Jewish parents of French descent, and moved to Paris in 1855 after a few years in Caracas.

But he wasn’t completely isolated when he arrived in London with his partner, Julie Vellay, and their two young children in December 1870. They stayed with relatives in the southeastern suburb of Norwood, and he socialized with Monet and other émigré artists at a central cafe run by a French wine merchant.

Pissarro, 40, was frustrated by his lack of commercial success, and his family was homesick. Vellay described the English language as a “succession of curious noises.”

London wasn’t all bad for them, though. It’s where Pissarro and Vellay married; where he met Paul Durand-Ruel, an art dealer who would sell his work for decades; and where he painted several canvases in his formative Impressionist style.

“Monet and I were very enthusiastic over the London landscapes,” he later wrote. “Monet worked in the parks, whilst I, living at Lower Norwood, at that time a charming suburb, studied the effects of fog, snow and springtime.”

Time travel

Pissarro lived near the Crystal Palace, a glass-domed exhibition space that epitomized Victorian Britain’s sense of modernity and had been moved to Southeast London from Hyde Park in the 1850s. But the painter, who worked outdoors in wooden clogs, was more interested in suburban scenes unfolding around the corner.

One of Pissarro’s early London paintings, “Fox Hill, Upper Norwood,” shows figures walking on a snow-dusted residential street. When my father-in-law, Alec, drove me there on a blustery December morning, we noticed that many of the same homes were still there.

The winter sky was the same mottled gray that Pissarro liked to paint (and which Cat, my long-expatriated wife, loves to hate). I was struck by how well his muted canvas still captured the area’s rolling hills and refracted sunlight.

Then we noticed two people wandering down the street holding a print of the same painting. What were the odds of that? It turned out they were also Pissarro groupies, searching the present for clues to the past.

“It’s just like time travel,” one of them, Libby Watson, told me. “It’s about the nearest thing you can get to it — isn’t it? — to look at the old buildings and imagine you were there.”

When Pissarro arrived in London, the city was still expanding in tandem with new railways. The train line he painted in 1871 had opened in 1865 to serve new suburban commuters, as well as tourists traveling to the Crystal Palace from Victoria Station, near Buckingham Palace.

In 1866 or 1867, my in-laws’ house was built beside the line on a street that had been a foot path through fields near the village of Dulwich, whose name derived from an Old English term for “the meadow where dill grows.” The street was in Forest Hill, a newish suburb that, like Norwood, took its name from the Great North Wood, an ancient forest that was mostly cut down as London barreled south in the 19th century.

Not everyone liked the pace of change. The Victorian art critic and social philosopher John Ruskin, who lived in the Dulwich area, complained that fields near his home had been dug up for building sites or cut by the “wild crossings and concurrencies” of railroads.

“No existing terms of language known to me are enough to describe the forms of filth, and modes of ruin,” wrote Ruskin, who left London in 1872 for England’s Lake District.

London’s 19th-century expansion was not well-organized but “higgledy piggledy,” as my father-in-law says, and fueled by railway rivalries. The line Pissarro painted was run by a company that battled a neighboring one for passengers. Both were run by “belligerent characters” who built unnecessary tracks for the sake of competing, according to the railway historian Christian Wolmar.

The competition “resulted in a complex and underinvested network that still causes distress to commuters today,” Mr. Wolmar wrote in “Fire and Steam,” his 2007 history of British railways. And as any Southeast Londoner will tell you, train service in the area remains notoriously patchy.

But for a visiting 19th-century Impressionist, it must have been fascinating to watch a giant city devour countryside in real time.

On the tracks

“Lordship Lane Station, Dulwich,” Pissarro’s 1871 train painting, shows a black engine belching smoke as it approaches the viewer on tracks that run through empty fields. A railroad signal — a metal or wooden contraption whose placement indicated whether a train driver should stop or go — hovers overhead in a horizontal position.

Today the scene is almost unrecognizable. The train line closed in 1954, nearly 18 years after the Crystal Palace burned down. Lordship Lane Station was later demolished, and a local bus route was extended to cover the former railway route.

Housing now sits on what was once open land, and the railway bridge Pissarro painted from lies in a nature reserve (and is temporarily closed for renovation).

The sliver of land where tracks once ran past my in-laws’ place has been turned into a nature path.

As for the canvas, it now hangs in central London’s Courtauld Gallery. When we visited in December, I was so busy trying to keep our toddlers from destroying priceless artworks that I didn’t get much of a chance to study it.

But we did get a flavor of Britain’s railway heritage at other points on our trip. One day we took our locomotive-obsessed boys on a steam train ride along the Bluebell Railway, a heritage line outside London. Those tracks were once owned by a railroad company that funded moving the Crystal Palace to Southeast London after the Great Exhibition of 1851.

The kids also played on trains at the London Transport Museum, where a display informed us that “unstructured” 19th-century growth had transformed the city.

“Lordship Lane” highlights the drama of that transition because Pissarro’s train tracks divide a patch of still-rural land from a newly suburbanized one, Karen Serres, the senior curator of paintings at the Courtauld, told me when I called for a chat.

And unlike many of Pissarro’s other works, “Lordship Lane” doesn’t show any people. When the Courtauld’s staff X-rayed the canvas in 2007, they discovered that a human figure had been painted in a corner of an early version, then painted over.

The train, then, is the main subject. And you can’t avoid it because it’s heading right for you.

The signal in question

“Lordship Lane” is often compared to “Rain, Steam and Speed,” an 1844 landscape painting by J.M.W. Turner. Pissarro and other French Impressionists openly admired English artists, whose work they saw in London’s museums. Art historians have long debated the extent to which the Impressionists were influenced by British painters.

I don’t have a strong opinion about that. But in London, I was very interested in settling another, even more arcane, historical debate.

Specifically, I had been told that “Lordship Lane” is the painting about which the Courtauld receives the most complaints. Among other things, critics apparently argue that Pissarro’s Victorian train signal should have been vertical for “go,” not horizontal for “stop.”

Dr. Serres told me what I’d heard was correct. Over the years, she had changed the museum’s description of the painting after railway enthusiasts flagged mistakes, including its original title of “Penge Station, Upper Norwood.”

But she had never known what to think about suggestions that the signal should be vertical for “go” because the train appears to be idling at the station. Her own impression was that the train was “lightly beyond” the platform and had already been given the signal to proceed. Then again, other details in the painting, including the station and the train smoke, did not look especially accurate.

“It’s very difficult to know how completely accurate these things are, and indeed that wasn’t his point,” she said. “It was to make a beautiful composition.”

My father-in-law said that he tended to think the signal was correct because the train seemed to have already passed the station. But he wasn’t totally sure.

So I called Mr. Wolmar, the author of “Fire and Steam,” who later emailed me to say that he agreed.

“The train is well past the signal so it will have reverted to default which is horizontal,” he wrote.