MI 79
| Company type | Electric multiple unit train |
|---|---|
| Industry | Rail transport |
| Founded | 1980–1985 |
| Founder | Alsthom and Société Franco-Belge |
| Successor | MI 84 (partially) |
| Headquarters | Paris , France |
Area served | Paris RER B line |
| Products | Dual-voltage electric multiple unit trains |
| Owner | RATP |
MI 79 (Class Z 8100)
The MI 79 (Matériel d'Interconnexion de 1979), also identified as Class Z 8100, is a dual-voltage electric multiple unit train designed for operation on the RER B line of Paris's Réseau Express Régional (RER), a hybrid system combining suburban commuter and rapid transit services. Conceived in the 1970s to address the RER B’s requirement for dual-voltage compatibility, these trainsets were manufactured by Alsthom and Société Franco-Belge between 1980 and 1985, resulting in a total of 120 sets (480 cars).
Constructed with aluminium-bodied cars, each four-car trainset spans 104.16 meters and can run on both 1,500 V DC and 25 kV AC overhead power supplies via pantograph collection. The propulsion system delivers around 2,824 kW through eight CEM/Oerlikon traction motors managed by analog chopper and thyristor controls. The maximum speed attainable is 140 km/h, supported by a combination of disc brakes, regenerative braking, and eddy current braking technologies.
After refurbishment completed between 2010 and 2015, 116 MI 79 trainsets remain operational under RATP management. These units are engineered for compatibility with MI 84 trainsets—initially created for the RER A line but later reassigned to RER B during the 2010s to improve fleet efficiency.
Primary maintenance facilities include depots at Massy and Mitry-Mory. Safety on the busy RER network is ensured through systems such as EAS, KCVP, and SACEM integrated into these trains.
History
The early 1970s marked the beginning of the Réseau Express Régional (RER) project, designed as a hybrid suburban and rapid transit system serving Paris and its surrounding areas. The RER B line was established by connecting two pre-existing railway lines through a newly constructed tunnel running north to south beneath central Paris.
This line presented operational challenges due to differing infrastructure: the southern segment, the Ligne de Sceaux owned by RATP, utilized 1.5 kV DC electrification with platforms at , whereas the northern segment, operated by SNCF’s Chemin de Fer du Nord, employed 25 kV AC electrification with platforms at .
To address these discrepancies, the MI 79 trainsets were developed to function across both electrical systems and featured pneumatically controlled exterior steps to accommodate platform height variations. Each four-car unit measured , allowing two coupled sets to fit within the shortest platform lengths of .
Since both SNCF and RATP jointly managed RER B operations, they collaborated on commissioning the MI 79 trains. The contract awarded in 1979 went to Alsthom and Société Franco-Belge (later absorbed by Alsthom). The initial order for MI 79A consisted of 72 four-car sets—31 allocated to RATP and 51 to SNCF—with first units arriving for testing in 1980. Subsequently, RATP procured an additional 38 MI 79B trainsets featuring enhancements such as simplified couplers, improved brake controls, and updated door mechanisms. All deliveries were completed by 1985.
Due to technical difficulties with the MS 61 trains, RATP reassigned 24 MI 79 sets from RER B to RER A to compensate for equipment shortages. During the harsh winter of 1985–86, insulation failures within MI 79 electrical systems caused further service interruptions, delaying the withdrawal of older Z 23000 railcars dating back to 1938.
One trainset sustained damage during the 1995 France bombings and another during the 1996 Paris RER bombing; undamaged cars from these incidents were subsequently combined into functioning sets.
From 2010 through 2015, the MI 79 fleet underwent a major refurbishment valued at €275 million aimed at extending their operational lifespan and enhancing reliability.
Design
The interior design of the MI 79 reflected the stylistic trends of its time, showcasing red and blue vinyl seating, cream-colored ceilings, yellow side panels, and dark blue walls at the car ends. Tailored for RER B operations to Charles de Gaulle Airport, these trains featured overhead luggage racks positioned above the seats.
One significant design drawback involved the flip-down seats located near the entry doors, which, when lowered, limited the boarding area to merely .
Built with aluminum car bodies, the MI 79 trainsets were relatively lightweight given their dimensions and the era of manufacture. Each four-car formation is equipped with eight motors—one on each axle of the motorized cars—providing power outputs of under DC and under AC electrical systems. This power-to-weight configuration allows acceleration from a 208-meter platform in 23 seconds and reaching maximum speed in 100 seconds.
Technical details
An MI 79 trainset consists of four vehicles: two motorized units flanking two unpowered trailer cars. The pantograph is installed on one of the trailer cars, creating pairs of motorized and trailer cars that function as separate half-trainsets.
The motor cars contain four DC motors (model EHO 3262 A) produced by Alstom in collaboration with CEM and Oerlikon. Each motor delivers and is air-cooled, mounted on bogie frames. Power regulation is handled through an analog TCO thyristor chopper; AC input is converted to DC for traction.
The braking system integrates disc brakes, dynamic braking, and eddy current brakes. Under DC operation, dynamic braking returns energy to the overhead lines, while under AC power, it dissipates energy as heat through resistors in a rheostatic setup.
Formations
As of , there were 68 MI 79 trainsets stationed at the Massy RATP depot and 48 housed at the Mitry SNCF depot.
Trainsets are organized in a 2M2T configuration, consisting of two motor (M) cars and two trailer (T) cars:
<-Mitry/Roissy CDG airport Robinson/Massy/St Remy->
| ||||||
| Car No. | 1 | > 2 | 3 < | 4 | ||
|---|---|---|---|---|---|---|
| Type | M | T | M | |||
| Numbering | ZBD 81xx(even number) | ZRAB 281xx(even number) | ZRB 281xx(odd number) | ZBD 81xx(odd number) | ||
- Cars numbered 2 and 3 each carry an AM 62 BU pantograph.
- Initially, car 2 contained both first- and second-class seating sections.
Refurbishment
From 2010 through 2015, a comprehensive €274.75 million overhaul was undertaken on 119 MI 79 trainsets operating along the RER B line. The refurbishment encompassed technical enhancements, installation of refrigerated ventilation systems, and modifications to interior layouts, thereby prolonging operational service life by approximately 15 to 20 years.
Following a prototype upgrade conducted by Alstom in Reichshoffen, subsequent refurbishments were carried out at RATP facilities in Sucy-en-Brie. By late 2013, modernization had been completed on 58 trainsets; this number increased to 100 by mid-2015, with final completion achieved in December that year.
In September 2011, asbestos traces were discovered within some refurbished units—specifically affecting between 32 and 36 out of the 60 inspected cars—which led to their removal from active service and resulted in a reduction of available trains on the line by roughly one-fifth.
Interior alterations included elimination of overhead luggage racks—which had previously hindered effective ceiling airflow—and replacement of flip-down seats near doors with standing bars. Additional luggage storage was created adjacent to doorways by removing a cluster of four seats. Other improvements involved upgraded lighting systems, added handrails, removal of internal partitions, and implementation of the SISVE passenger information system that provides audio announcements alongside visual displays indicating upcoming stops.