The RER B line is a regional rapid transit line that cuts through Paris in tunnels from north to south, connecting the northern and southern suburbs and also two major airports, Charles de Gaulle in the north and Orly (via a branch line) in the south.
From north to south, the RER B trains have seven stations in Paris: Gare du Nord (North Station), Châtelet-Les Halles, St-Michel-Notre-Dame, Luxembourg, Port-Royal, Denfert-Rochereau and Cité Universitaire.

An RER B train in Luxembourg station, Paris
To get to Massy I took a southbound RER B train from Luxembourg, near my hotel. Since not all the trains stop at all the stations, there are large signs with lights that light up for each train, showing exactly which route it will take and where it will stop. For the train in my photo, seventeen stations are lit up, but not the one I wanted, so instead of boarding this train I just took a photo of it. For the next train, about two minutes later, the station Les Baconnets did light up, so that’s the one I took.

RER station Les Baconnets
The closest RER station to the Massy Opera is Les Baconnets, which is actually not located in Massy but in the adjoining town of Antony. From Les Baconnets it is only a fifteen-minute walk to the opera house via Avenue de la Fontaine Mouton, Avenue de Bourgogne, Rue de Montpellier and Avenue de France (avoiding the big construction site in front of the opera). It would also be possible to take the number 119 bus from Les Baconnets to Place de France, but I preferred to walk.

Construction site in front of the opera house
When I visited Massy in 2016 there was a large construction site directly in front of the opera house called Franciades-Opéra, designed to create a new commercial and residential center for the city.
Apparently the square in front of the opera, called Place de France, has up to now been mis-used as a huge open-air parking lot. Now the cars are being banished underground and the whole neighborhood (which dates originally from the 1950s) is being redeveloped.

Construction site as seen from the opera house
Another major project in the coming years will be (if all goes well) the development of the new Grand Paris Express system of automatic métro trains, which will include a station called Massy Opéra directly behind the opera house.
This proposed new Grand Paris Express network is intended to supplement the existing RER (Réseau Express Régional) lines by forming a wide circle of train lines through the outer Paris suburbs, to facilitate train travel throughout the region without forcing people to travel into Paris and then back out again.
My photos in this post are from 2016. I revised the text in 2018.
See also: Opera in Massy.