The MS 61 (French: Matériel Suburbain de 1961, English: Suburban rolling stock of 1961) was an electric multiple unit trainset that was operated on line A and line B of the Réseau Express Régional (RER), a hybrid suburban commuter and rapid transit system serving Paris and its Île-de-France suburbs.

The first MS 61 trains (type A units) entered service on the Ligne de Sceaux (now the southern part of RER B) from 29 June 1967.

[3] Type C units entered service when the western end of the RER A between La Défense and Charles de Gaulle–Étoile opened on 19 January 1970, and reached Auber on 23 November 1971: type D units then reinforced the former when that line extended to Saint-Germain-en-Laye on 1 October 1972, displacing the SNCF Class Z 1400 [fr] trains.

For the remainder of their service life, MS 61 trains operated on the RATP-owned sections of the RER A, which (from 1 April 1992) was from Saint-Germain-en-Laye to Boissy-Saint-Léger and (occasionally) Marne-la-Vallée–Chessy.

[7] In 2014, the RATP appointed Veolia Environment to dismantle and recycle the MS 61 units that were part of the second refurbishment programme, plus two additional cars.

[8][9] In 2010, the RATP selected trainset 24 (cars M.15050, AB.18024 and M.15151) for preservation at as part of the historic rail vehicle collection at the Villeneuve-Saint-Georges yard.

[4][10] According to the RATP, one MS 61 unit was capable of carrying up to 721 passengers (292 seated and 429 standing): the trains also provided luggage racks in both first and second classes, but they were removed in the first refurbishment programme from 1985 to 1992.

During the first refurbishment programme, they were replaced by a fire and vandal-resistant design that used a navy blue plastic frame and purple fabric covering.

In the second refurbishment programme, the seats were replaced again, with a similar design that used an orange plastic frame and a moquette of multicoloured stripes.

[15] According to French railway magazine Historail in 2016, the crash was caused by human error relating to a newly installed signal that came into operation two days prior.