Take the B train - A day crossing Paris by train.

France’s capital viewed from a suburban railway.

The Economist, December 19, 2017.

FROM SAINT-REMY-LES-CHEVREUSE TO AEROPORT CHARLES DE GAULLE.

Main line : Saint-Rémy-lès-Chevreuse—Courcelle-sur-Yvette—Gif-sur-Yvette—La Harquinière—Bures-sur-Yvette—Orsay-Ville—Le Guìchet—Lozère—Palaiseau-Villebon—Palaiseau—Massy-Palaiseau—Massy-Verrières—Les Baconnets—Fontaine-Michalon—Antony—La Croix de Berny—Parc-de-Sceaux—Shunt : Sceaux—Fontenay-aux-Roses-Robinson—Main line : Bourg-la-Reine—Bagneux—Arcueil-Cachan—Laplace—Gentilly—Cité-Universitaire—Denfert-Rochereau—Port-Royal—Luxembourg—Saint-Michel-Notre-Dame—Châtelet-Les Halles—Gare du Nord—la Plaine-Stade de France—la Courneuve-Aubervilliers—Le Bourget—Drancy—Le Blanc-Mesnil—Aulnay-sous-Bois—Shunt : Sevran-Livry—Vert-Galant—Mitry-le-Neuf—Villeparisis—Mitry-Claye—Main line : Sevran Beaudottes—Villepinte—Parc des Expositions—Aéroport Charles de Gaulle 1— Aéroport Charles de Gaulle 2.

On sunny weekends Saint-Rémy-lès-Chevreuse bustles with visitors. They admire its goats and cows, buy fresh farm eggs, picnic in riverside meadows and explore the 11th-century hilltop castle. Furtive mushroom-pickers slip to secret spots in the woods nearby.

Parisians have been enjoying this fantasy of rural life since the railway first arrived in St-Rémy 150 years ago. The line was dubbed the “lilac train” in honour of the mauve bouquets taken back to the Smoke as remembrances of day-trip idylls.

The lilac line is now, rather less charmingly, line B of the regional express network (RER, in French). On maps it appears as a blue thread with frayed ends stretching diagonally across Paris from bottom left (St-Rémy) to top right. It links country, leafy suburbs, the opulent city, tourist delights (and traps) and grotty northern banlieues. At its far end is Charles de Gaulle airport, a sprawling avant-garde mess that touched down on the fields of sugar beet and tulips next to the village of Roissy in 1974.

Despite its name, the B is no express. Tracing its 80km (50 miles) takes an hour-and-a-half—not that anyone normally does so. Day-trippers go out; commuters come in; people travel a few stops for shopping, or to see friends, or to go to school; at airports (the line indirectly serves one at Orly, as well as Charles de Gaulle) and railway interchanges travellers from further afield pile on. For all of them, the line is a means, not an end—and not a particularly appealing one. Many of the carriages that between them provide almost a million journeys a day wobble disturbingly along, with yellow-painted walls, dim lights, metal coat-hooks over threadbare purple seats, a crackling tannoy, doors that hiss, brakes that squawk like gangs of novice clarinettists. Strikes or breakdowns, both frequent, bring enormous disruption.

Approach the line as an end-to-end experience, though, rather than a sometimes irritating bit of infrastructure separating the A you are at from the B you desire, and things become more fascinating. François Maspero, a leftist writer, and Anaïk Frantz, a photographer, celebrated this when they spent a month on the RER B in 1989. Their aim was to show warm, welcoming human life in the poorest suburbs and defy those who imagined a “circular purgatory, with Paris as paradise in the middle”. Their journey told them something about Paris—but also something universal.

The railway lines that rush, or chug, into the world’s cities all give you that universal: the constant intermingling, and disentangling, of rich and poor, local and visitor, native and migrant, young and old; the enriching diversity and grating inequality. But each line also gives you something specific to each particular string of places, be it the A train in New York, running from JFK airport up to Harlem, the Yellow Line that runs through Delhi’s wealthy centre and crowded slums on to its enormous, sprawling suburbs, or London’s District Line, which runs from bankerly Richmond to working-class, immigrant-heavy East Ham. For the particulars of Paris, take the B train.

RERing to go.

Well before dawn on a weekday morning a fingernail moon hangs over St-Rémy. Birds have their voices, but little else stirs. The windows of Le Chalet Caffe, a bistro and tobacconist, are dark. At neighbouring La Giostra, where last night women in tweed jackets ate smoked-salmon pizza, the doors are blocked by overflowing bins. Only the station is bright, its four faux-Victorian lamps ablaze, white floodlights criss-crossing the platform with shadows.

“Yes, yes, don’t worry yourself,” the driver says amiably as he walks to the cab. “The only direction is Paris”. A mother waves off her shaven-headed son, a military-camouflage pack on his back. A middle-aged woman scowls at a pair of guide books for Dubai.

The first passengers—ruddy-faced, well-to-do, lugging suitcases—appear to be poised for travel, not commuting. They are almost silent. Those who get on respect a universal rule of public transport in the West, sitting as far as possible from their fellows (they avoid eye contact, too). The line loops past a ruined farmhouse. There were vineyards down here, before phylloxera struck.

At Bures-sur-Yvette, as darkness begins to lift, potted geraniums cram windowsills and the 19th-century villas boast turrets. Haydn and Mozart will be performed next Saturday in Saint Matthieu’s church, which predates them both by centuries. A white-haired man in a linen jacket, baguette in his wicker basket, heads into the “artisanal” butchers. Take coffee and a croissant at the bistro—the owner proposes traditional “tartiflette” for lunch—and nothing seems to have changed for decades. A few Portuguese-speakers in hard hats stand at the aluminium bar; the rest of the clientele is pale-skinned. The village memorial counts 67 villagers lost in two world wars.

As dawn gives way to daylight the train slips under a 19th-century aqueduct on millstone pillars, built in the place of a Roman predecessor, and enters the suburbs proper. Station walls are draped in purple ivy. Orange-red tiles top many homes. Posters promote exhibitions in Paris. Near Sceaux the tree-lined avenues remain, as they have been for more than a century, a home for writers, academics, musicians (as well as disturbingly well-coiffured dogs). When Erik Satie died in Arcueil, his bedroom was said to contain 100 umbrellas and an untouched piano.

This is a well-heeled world for the cultured and comfortably liberal. Its voters flocked to the young centrist Emmanuel Macron in last spring’s elections. Not all is genteel. You can walk by large housing estates in the southern banlieues, just as in the north. But they are for the most part well tended, fenced off, freshly painted. Once in a rare while the graffiti, ubiquitous along the line, is actually uplifting. At a station beside a stately school, a wall enjoins travellers to “hope, hope”; the words are English, the lower-case letters in the looping, cursive hand that French children must still perfect. It is a different story north on the line. Fresh graffiti in the station at La Courneuve says “A bas Le Macronisme” (Down with Macronism). Up there, the voters turned to hard-left politicians, such as Jean-Luc Mélenchon. Up there, food outlets offer fried chicken, not tartiflette.

As suburb becomes city in the south, most passengers are clearly on their way to work. The train breathes in tieless young men in dark suits watching Les Simpson on their phones; middle-aged ones playing Candy Crush. A man in a black T-shirt studies a slender book on quantum gravity; perhaps he is from the nuclear-research centre at Saclay.

For some, the train itself is a workplace. A silent woman in a leopard-print headscarf hands out yellow cards describing four jobless brothers. The pale blue card of a pot-bellied man reports he has no employment and wishes you safe travels. The beggar in the dark blue cap is readiest to smile.

Brass bands play and feet start to pound.

The line only reached the heart of Paris in 1895, when a tunnel was built from Denfert to Luxembourg station. It took a further eight decades for it to be coaxed under the River Seine (down an especially steep incline) and connected with the north.

At the trio of underground stations—Port-Royal, Luxembourg, Saint-Michel-Notre-Dame—which constitutes the heart of tourist-Paris, couples with small backpacks flood the train. They are insistently warned by loudspeaker, in several languages, of the dangers of pickpockets. With the carriages too packed to pass through, begging moves to the platforms and corridors. In the monstrous maze of Châtelet Les Halles—120 trains an hour, and allegedly the largest metro station in the world—women in black and children in bright coats pass entire days sitting underground on tiled floors. One woman explains her home was in Aleppo.

There are buskers, too. The regional rail operator, RATP, says there are 300 or so officially accredited ones. Antoine Naso, a stocky former line manager, calls the city’s 5.5m Metro passengers “one of the biggest music scenes in the world”; he recently put on a concert of some of the best acts of the past two decades. Every few months Mr Naso auditions new talent in a tiny basement mock-up of a metro tunnel. Two choral singers in sensible dresses perform Henry Purcell’s “Let us wander”. Endless young men with beards cover rock and pop anthems. The stand-out is Podkopaev Evguenio Korela, once a Finnish hard-rocker, who has by some technical magic transformed the lime-green handle of a broom into a funky percussive synthesiser. He has played in the city for 30 years; he talks movingly of his love for the “liberalism [and] democracy of Paris”.

Mr Korela won’t perform on the B train. It is too noisy. He and bigger bands like Eurofolk, an eight-piece Ukrainian gypsy-jazz group with quite a following, prefer the acoustics and space of stations like Châtelet Les Halles; the echoes of Eurofolk’s set can be heard from the RER platform. But Wayne Standley, a midwestern folk guitarist, says he has played the length of the B line. The southern part “got lonesome towards the end,” he recalls, “but you needed a rest. And it was quieter, so you could sound a little better”. He remembers a Malaysian guitarist friend who thrived on the B train with a set consisting mostly of advertising jingles.

North of Gare du Nord, the line threads between massive residential blocks and post-industrial districts, some buzzing, some distinctly not. As the steelworks, glyceride refineries, car plants and warehouses up here have closed, they have been replaced by studios for film editors and artists, glass office towers, shopping malls and recycling plants. The beggars return to the carriages—but up here they are mostly white, and addicts. A grinning woman in a black baseball cap moves quickly along on her crutches, a carriage per stop; she offers no story, just asks for money.

Life here is far tougher than in the centre or south. In 2010 Emmanuel Vigneron, a geographer,compared death rates, medical care and incomes of people near every station on the RER B, adapting a technique first used to study health-care differences along the belt road of Tahiti. “We showed that by moving 15 minutes out of Paris mortality rates would double”, he says. In a sample of women from Port-Royal, in central Paris, and La Plaine-Stade de France, where the average income is dramatically lower, he found that those few stops further north represented an 82% higher risk of dying in a given year. Similar studies for commuter lines elsewhere have since become a headline-friendly way for various cities to worry about their inequality.

The presiding spirit of the banlieues is Paul Delouvrier, an administrator ordered by Charles de Gaulle to sort the “shambles” of the outer city and its slums. His legacy is one of huge, dispiriting tower blocks—the main focus of the book Maspero and Ms Frantz produced in 1989. For all their work’s sympathetic, humane charm, though, the decades since have mostly strengthened a popular view of northern banlieues as alien and threatening, a home to Islamist and immigrant extremists, sites of police brutality, riots, drug-dealing and rape. In February 2017 riots raged for several nights in estates near the RER station at Aulnay-sous-Bois after police reportedly sodomised a young man with a baton.

Take an afternoon walk around estates such as La Courneuve 4000 or Aulnay 3000, where thousands of people are packed tightly together, and it is no wonder that many told Maspero that their dream was to move to a pavillon, one of the modest, privately owned villas that predated the projects. A massive wall of apartments has a broken plastic symbol of a galleon on its side. It is said to be due for demolition soon. By a doorway a woman tends a fire in a shopping trolley, roasting maize cobs to sell. Men stand on the roadside with tiny coolboxes, offering single cans of fizzy drink. Above them is an enormous poster of Moussa Sissoko, a footballer in the national team who was a resident here from 1989 to 2001. It is a reminder that some, at least, succeed and leave.

Get off at Drancy station and things look more welcoming. But among the grey, four-storey blocks of La Muette estate, built in the 1930s, there is a wooden cattle wagon, stencilled with a Star of David on one door as well as lettering of SNCF, France’s national railway company. Parties of schoolchildren circle the site with a guide. In the second world war La Muette, surrounded by barbed wire and watch-towers, was Paris’s concentration camp for those of Jewish descent. A memorial centre at the site, inaugurated by France’s president only in 2012, explains how mostly French guards oversaw the camp and the transport of inmates to the border, en route to Auschwitz. Today, bicycles are scattered on communal grass in early-evening sunshine.

All passed out of our lives.

On the last leg north, the city breathes out. Having sent its commuters home it has a few hours to relax. Four teenage boys, sweaty in football kits, noisily debate their performances on the pitch. Young women run laughing and cheering into the carriage: one wears an ankle-length, dazzling white fur coat and sports a beehive of red hair; the head of her friend is shaved in an intricate criss-cross pattern.

The line splits at Aulnay. One branch slips under a motorway and out into fields of maize. The other takes a jumble of commuters and travellers towards the airport. In a few years, though, the travellers will be gone. Before the Olympics come to Paris in 2024 a non-stop service will start whisking passengers from Charles de Gaulle to Gare de l’Est, in the centre, in just 20 minutes. The locals will be left alone.

Monks in saffron robes and orange bobble hats stare out at arenas, hotels, exhibition centres and construction sites, approaching a terminal that is floodlit in red, white and blue, a top-heavy mass of concrete and shadow. Its signs are algebra in blue—2A-G divided by T3 in this direction, but carry T1 into the shuttle-bus column. Silvery escalators rise from the platform, their passengers’ heads tilted upwards—a group of Chinese tourists, each with a rectangular cardboard train ticket in hand; those monks, unencumbered by any luggage at all. It is near midnight as they glide up, and up, into the artificial light. Below, the windows of the B train, etched with graffiti, are radiant in the gloom.

This article appeared in the Christmas Specials section of the print edition under the headline "Take the B train"