It was in this context that a railway line linking Paris to Saint-Germain-en-Laye was envisaged by the Pereire brothers, who requested a concession for its construction in 1832.

[1] The success of the railway line from Saint-Étienne to Lyon[2] quickly made the national government aware of the importance of developing this new mode of transport.

It therefore seemed essential to build a line from the capital, to make this new means of transport known to the public, and thus arouse the interest of politicians and financiers.

[5] The line was to connect the capital to Saint-Germain-en-Laye, a popular Sunday stroll spot for Parisians, whose proximity to Paris limited the investments required.

[7] The route to Le Pecq, 19 km (12 mi) long, is located on the plain and, apart from two river crossings, presented few construction difficulties.

François Arago declared: "I affirm without hesitation that in this sudden passage, people subject to sweating will be inconvenienced, that they will gain chest infections and pleurisy".

[10] Beyond Clichy, the route, in a sector that was still relatively undeveloped, posed fewer expropriation problems than in the immediate and more urbanised surroundings of Paris; it crossed fields and forests for most of this final section.

It is above all a source of prosperity for the commune of Batignolles-Monceaux; all the inns on the Barrière Monceaux are crowded, at meal times, with the five hundred workers who work in the Paris underground and in the earthworks of the Batignolles plain.

The movement of curious onlookers, who travel to the plain to see the manoeuvres of the wagons, on the temporary railways already established over a great length, maintains an extraordinary activity in the commune."

The inauguration of the line takes place on 24 August 1837 in the presence of the royal family and in particular Queen Maria Amalia, but in the absence of King Louis-Philippe.

[6] In the Journal des débats politiques et littéraires, Jules Janin raved: "Yesterday, going to Saint-Germain was a journey; today it is just a matter of leaving one's house."

L'Écho français appreciated the technical feat ("We were struck to the highest degree by the magic of this communication, so rapid and so to speak instantaneous"), while putting it into perspective: "but the eccentricity of the points of departure and arrival make it an object of curiosity and exhibition rather than of utility and exploitation."

[13] Listed on the stock exchange, the share price of the company operating the line from Paris to Saint-Germain quickly doubled: it rose to 1,072 francs in 1838.

[14] The development in England of atmospheric railway technology made it possible to envisage building a steep extension from Le Pecq to Saint-Germain.

[15] On 10 September and 20 October 1844, agreements were signed between the Minister of Public Works and the Compagnie du chemin de fer de Paris à Saint-Germain for the test to take place between Nanterre and the Saint-Germain plateau, in return for an extension of the line between the terminus of Le Pecq and said plateau.

The terminal station was built in a trench in the château park, breaking the symmetry of Le Nôtre's flowerbeds in the process, but without apparently provoking any protest.

[13][18] On the way there, the ascending track has a cast iron tube 63 cm (25 in) in diameter, split at its top, but made watertight by two leather lips.

It contained a piston attached to the chassis of a steering wagon, spreading the lips of the tube which closed after its passage, allowing it to be sucked in and made to climb the slope.

[13] On the way back, the train descended by simple gravity to Le Pecq, where the steam engine from the outward journey awaited to pull it to Paris.

The system works as best it can, but rapid technical progress with the arrival of more powerful locomotives means it is abandoned in 1860 for a classic steam traction by simple adhesion.

From 3 July 1860, a locomotive[20] of class 030 is placed at Le Pecq at the end of the train and provides the push to assist the leading engine.

All were created by separate companies, which had to run their trains on the same tracks, despite successive additions, and coexist in the same terminal station, which posed increasing operational problems.

Thanks to its many lines finely serving the western suburbs of Paris, the Saint-Lazare station then became the most important in the capital in terms of its traffic, which doubled every twenty to twenty-five years for more than a century.

The integration of the section from Nanterre University to Saint-Germain-en-Laye required major adaptation work, in particular re-electrification by catenary using 1,500 volts DC for the circulation of MS 61 rolling stock.

[28] In 2017, during preventive archaeological excavations as part of an urban planning project by the Institut national de recherches archéologiques préventives (INRAP) researchers, the remains of the base of the original Pecq station, the former terminus of the line in 1837, were unearthed.

During eleven weeks of meticulous research on approximately 1,600 m2 (17,000 sq ft), the archaeologists uncovered not only building structures, but also the turntable for reversing locomotives, buttons belonging to the staff, as well as ceramic tableware decorated with gilding, used in the station restaurant.

From Paris to Nanterre-Université, the line is electrified like the entire Saint-Lazare network at 25 kV-50 Hz single-phase,[31] equipped with Block automatique lumineux (BAL), a form of automatic block signaling using coloured light signalling,[32] Contrôle de vitesse par balises (KVB; speed control by beacons)[33] and a ground to train radio link without data transmission with identification.