The "premetro" lines are powered (like the ground-level Brussels tram lines) by overhead lines at 600 V DC; the conventional metro lines use instead an elevated third rail at 900 V DC. They all use standard gauge. Conventional metro platforms are "high platforms", built flush with the floor of the metro compartments; "premetro" platforms are the same height, but with a lowered section at least as long as the longest tram, for compatibility with tramways which must be able to take passengers from the sidewalk of a street, or even from the street floor itself. Upgrading a line from "premetro" to metro service includes, among others, raising the whole platform to metro height and replacing the overhead line by a third rail power supply.