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The principle behind the rubber-tired metro
At left : The first ever rubber-tired parisian Métro at the Porte-des-Lilas station.

Rubber-tired Métros were first invented by the Régie Autonome des Transports Parisiens in order to allow better acceleration peformance so to increase the service frequency, as well as minize noise and vibrations towards neighbouring buildings.
At right : One of the very first "Michelines" was the "Lafayette", built in stainless steel by Budd in the U.S.A. in 1932 to be exported to France.

With the experience gained on main line railroads on both sides of the Atlantic with the famous Michelines, the legendary rubber-tired light trains, the RATP started in 1951 to experiment with a rubber-tired motor, the "151". The experiment occured on the Porte-des-Lilas_Pré-Saint-Gervais "shuttle", which was closed to the travelling public since 1939 and was only used for motorman training.

At left : A Micheline truck full of axles (tires are not yet mounted) Notice the engine and gearbox transmission mounted within the truck framework.

For the record, the Michelines ran on a classic track, and given the small rail-tire contact surface and low axle load permitted by the pneumatic tires, they had to have a high number of axles for a reduced load. But the Michelines did not last long despite their well-cushioned ride, since they were particularly sensitive to track defects that could result in... flat tires...

At right : The Reading Railroad ordered one...

But what was bad for mainlines wasn't necessarly so for urban subways stuck with a tremenduous traffic increase since the end of the war... The captive rolling stock and urban subway network constraints allowed the RATP to use a axle count reducing solution, by using special rolling stock rolling on a specially modified track.

At left : ...then the Pennsylvania Railroad had one...

Instead of going on the metal rail, wider tires rest on a new track on the outside of the classic rails that are nevertheless retained to allow classic rolling stock to go, and in case of a flat tire to take over from the failure of a tire, since the rubber-tired cars keep their classic metal rail wheels. Direction is maintained by using horizontal guide wheels that roll against guide rails that double as power supply feed lines.

Direction control at switches is effected by the extended metal wheel flanges that rub against the switch points. Originally in Paris, the main tire rollways were removed at switches, but in Montréal, the main tires are bearing the weight of the trains on the switches.

At right : ...and then the Texas & Pacific Railroad too, in 1933. But the latter used the rubber tires only on the trailer, the motor having classic metal wheels.

At left : A MP55 series train on Paris line 11 at the Hôtel-de-Ville station.

Fully satisfied with experimenting with the M151, the RATP started by converting line 11 (Châtelet_Mairie-des-Lilas) to rubber-tired operation, and then to convert lines 1, 4 and 6. But the time required to convert a line and advances made in suspensions, electric traction control systems and full motorization (powering all the axles of a train) diminished the advantages of a rubber-tired Métro, and Parisian Métro line conversions gave way to newer steel wheel rolling stock.

In fact, only new Métro systems (Lyon, Marseille, Montréal, Mexico, Monterrey) were done using rubber tired trains, as converting existing systems has been proven uneconomic.

At right : A parisian truck seen from above. Notice the two axles with differential gears and their traction motors.
Note the classic rail steel wheels with their brake shoes, and the horizontal direction wheels.
The traction current pickup shoes are cozily mounted between the classy whitewall tires...
In Montréal, the brakeshoes are only mounted betwen the axles, inside.

Above: A MP-55 train on line 11. Notice the horizontal direction wheels in front of the truck.

Above: A MP-55 train at the Porte-Maillot station. Note that there is no guide bar so we can see how the direction wheels are positioned on the trucks.
Track and wheels schematics

Legend :

A Main tire
The full weight of cars as well as the whole tractive effort is normally borne by the main tires. For fire resistance reasons and to avoid corrosion of the radial frame, they are inflated with nitrogen.
A' Concrete rollway
The main tires run on this track, which is directly fastened to the tunnel floor. At switch points and frogs, they are made of metal.
B Guide wheels
Those smaller tires are mounted horizontally and guide the trucks and cars on the track by resting against the guide bars.
B' Guide bars
They both guide the trucks on the track and provide the traction current. They are continuously supplied with power (they are, in essence, the third rail) and are supported by insulators (D). The voltage in Montréal is 750 volts.
C Safety wheel
In case of a flat tire, they get down and contact the metal rail. In switches, the points touche the flanges and guide the trucks on the lined-up route. They actually have an extended flange as pictured so to keep contact with switch points when the bottom of the wheel does not touch the top of the rail.
C' Metal rail
A run-of-the-mill railroad track, down to the stantard gauge (1.435m - 4' 8" 1/2). They are both a safety rail in case of flat tires and are used by some work equipment; they are finally used as the current return path.
D Guide bar insulators
They electrically insulate the guide bars and support them and the lateral efforts to keep the trains on the track.
E Rail insulators
They electrically insulate the metal rails that carry the current return, and bear the full weight of rolling stock that rolls on them.
Metal rails are electically insulated from the tunnel floor, to avoid stray current and their galvanic corrosion problems. They are NOT considered as grounded, and therefore personnel is instructed to consider them with the same attention afforded to the guide rails.
F Lateral pickup shoe
They take the traction power from the guide bars.
G Return shoes
They return the traction power to the metal rails.
H Flat tire detector
When a tire is deflated and the safety wheel contacts the metal rail, the flange goes down very low and mechanically contacts the detection limit switch which, once triggered, sends the alarm signal.

Big thanks to Mr. S. Gambourg & M. Walton for correcting small errors and omissions.
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