In its first decades of operation, the trains were steam-drawn; even after the electrification of large parts of the network, a number of lines remained under steam. Today, the term S-Bahn is used in Berlin only for those lines and trains with third-rail electrical power transmission and the special Berlin S-Bahn loading gauge. The third unique technical feature of the Berlin S-Bahn, the automated mechanical train control[clarification needed], is being phased out and replaced by a communications-based train control system specific to the Berlin S-Bahn.