European electrification is mostly 25 kV AC at 50 Hz (primarily in Eastern Europe). Conversion to this voltage/frequency requires higher voltage insulators and greater clearance between lines and bridges and other structures. This is now standard for new overhead lines as well as for modernizing old installations. Denmark, despite bordering only 15 kV territory decided to electrify their mainline railways at 25 kV 50 Hz for that and other reasons. Simple European standardization with an alignment of voltage/frequency across Europe is not necessarily cost-effective since trans-border traction is more limited by the differing national standards in other areas. To equip an electric locomotive with a transformer for two or more input voltages is cheap compared to the cost of installing multiple train protection systems and to run them through the approval procedure to get access to the railway network in other countries. However, some new high-speed lines to neighbouring countries are already intended to be built to 25 kV (e.g. in Austria to Eastern Europe). Newer locomotives are always built with asynchronous motor control systems that have no problem with a range of input frequencies including DC. However the Deutsche Bahn train operator does still use older models from the standard electric locomotive series—even though some are now as much as 50 years old. As soon as these obsolescent models are decommissioned, it will be easier to standardise, but this may take a few decades to happen. Meanwhile, the Deutsche Bahn tends to order train sets that are capable of running multiple electrification systems. Distribution networks In Germany (except Mecklenburg-Western Pomerania and Saxony-Anhalt), Austria and Switzerland, there is a separate single-phase power distribution grid for railway power at ; the voltage is in Germany and Austria and in Switzerland. This system is called the centralized railway energy supply. A separate single-phase power distribution grid makes the recuperation of energy during braking extremely easy in comparison with 25kV 50 Hz system tied to 3 phase distribution grid. In Sweden, Norway, Mecklenburg-Western Pomerania and Saxony-Anhalt, the power is taken directly from the three-phase grid ( at ), converted to low frequency single phase and fed into the overhead line. This system is called the decentralized (i.e. local) railway energy supply. Generation and conversion The centralized system is supplied by special power plants that generate (or in the Swiss system) AC at and by rotary converters or AC/AC converters that are supplied from