 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.