Rubber-tyred metro

VAL tracks on the Lille Metro

Sapporo Subway guide rail and flat steel roll ways

Trains are usually in the form of electric multiple units. Just as on a conventional railway, the driver does not have to steer, with the system relying on some sort of guideway to direct the train. The type of guideway varies between networks. Most use two parallel roll ways, each the width of a tyre, which are made of various materials. The Montreal Metro, Lille Metro, Toulouse Metro, and most parts of Santiago Metro, use concrete. The Busan Subway Line 4 employs a concrete slab. The Paris Métro, Mexico City Metro, and the non-underground section of Santiago Metro, use H-Shaped hot rolled steel, and the Sapporo Municipal Subway uses flat steel. The Sapporo system and Lille Metro use a single central guide rail only.[4]

On some systems, such those in Paris, Montreal, and Mexico City, there is a conventional 1,435 mm (4 ft 8+1⁄2 in) standard gauge railway track between the roll ways. The bogies of the train include railway wheels with longer flanges than normal. These conventional wheels are normally just above the rails, but come into use in the case of a flat tyre, or at switches (points) and crossings. In Paris these rails were also used to enable mixed traffic, with rubber-tyred and steel-wheeled trains using the same track, particularly during conversion from normal railway track. The VAL system, used in Lille and Toulouse, has other sorts of flat-tyre compensation and switching methods.

On most systems, the electric power is supplied from one of the guide bars, which serves as a third rail. The current is picked up by a separate lateral pickup shoe. The return current passes via a return shoe to one or both of the conventional railway tracks, which are part of most systems, or to the other guide bar.

Rubber tyres have higher rolling resistance than traditional steel railway wheels. There are some advantages and disadvantages to increased rolling resistance, causing them to not be used in certain countries.[1]

Compared to steel wheel on steel rail, the advantages of rubber-tyred metro systems are:

• Faster acceleration, along with the ability to climb or descend steeper slopes (approximately a gradient of 13%) than would be feasible with conventional rail tracks, which would likely need a rack instead.[lower-alpha 1]
  • For example, the rubber-tyred Line 2 of the Lausanne Metro has grades of up to 12%.[5]
• Shorter braking distances, allowing trains to be signalled closer together.
• Quieter rides in open air (both inside and outside the train).
• Greatly reduced rail wear with resulting reduced maintenance costs of those parts.

The higher friction and increased rolling resistance cause disadvantages (compared to steel wheel on steel rail):

• Higher energy consumption.
• Worse ride, when compared with well-maintained steel-on-steel systems.[6]
• Possibility of tyre blow-outs - not possible in railway wheels.
• Normal operation generates more heat (from friction).
• Weather variance. (Applicable only to above-ground installations)
  • Loss of the traction-advantage in inclement weather (snow and ice).[lower-alpha 2]
• Same expense of steel rails for switching purposes, to provide electricity or grounding to the trains and as a safety backup.[lower-alpha 3]
• Tyres that frequently need to be replaced; contrary to rails using steel wheels, which need to be replaced less often.[lower-alpha 4]
• Tyres break down during use and turn into particulate matter (dust), which can be hazardous air pollution, also coating surrounding surfaces in dirty rubber dust.[7]

Although it is a more complex technology, most rubber-tyred metro systems use quite simple techniques, in contrast to guided buses. Heat dissipation is an issue as eventually all traction energy consumed by the train — except the electric energy regenerated back into the substation during electrodynamic braking — will end up in losses (mostly heat). In frequently operated tunnels (typical metro operation) the extra heat from rubber tyres is a widespread problem, necessitating ventilation of the tunnels. As a result, some rubber-tyred metro systems do not have air-conditioned trains, as air conditioning would heat the tunnels to temperatures where operation is not possible.