Overview tracks on the
Lille Metro 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. On some systems, such those in Paris, Montreal, and Mexico City, there is a conventional
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.
guide rail and flat steel roll ways 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. • For example, the rubber-tyred Line 2 of the
Lausanne Metro has grades of up to 12%. • Shorter braking distances, allowing trains to be
signalled closer together. • Quieter rides in open air (both inside and outside the train). • Possibility of tyre blow-outs - not possible in railway wheels. • Higher cost of maintenance and manufacture. 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. ==Similar technologies==