Poland considers 25 kV AC conversion of existing 3 kV railway lines

Poland's existing electrified railway network operates on 3 kV DC, a system also used on conventional railway networks in Italy, Belgium and Slovenia, as well as parts of Czechia and Slovakia. While modern multi-system locomotives can operate under different voltages, much of the older rolling stock remains restricted to a single traction power system. PKP PLK's proposed programme introduces a new element into Polish railway planning: the possible conversion of existing electrified main lines, rather than simply extending overhead wires to routes currently operated by diesel traction. The first phase, covering the period until 2035, includes electrification projects already under development, closing gaps between electrified lines and a pilot application of 2×25 kV AC. A second phase, from 2036 to 2045, envisages further deployment of the AC system and preparations for re-electrifying selected main lines. PLK has not yet identified which existing routes could be converted or published a network-wide timetable for replacing 3 kV DC. Why 2×25 kV instead of 3 kV DC? The existing 3 kV DC system requires high electrical currents to deliver the power demanded by modern locomotives and high-speed trains. This creates constraints related to voltage drops, transmission losses and traction substation capacity, particularly on heavily used lines. A 2×25 kV AC system uses an autotransformer arrangement with a contact wire at 25 kV relative to the rails and a separate feeder at the opposite voltage. The potential difference between the two conductors is 50 kV, but trains continue to receive conventional 25 kV AC through their pantographs. This arrangement allows higher-power transmission over longer distances with lower losses and fewer main traction substations. From the rolling stock perspective, it is compatible with vehicles equipped for 25 kV AC operation, subject to normal infrastructure compatibility requirements. The technology is already used on Italian high-speed lines and is planned for Poland's new high-speed railway network. Poland's high-speed railway will introduce the AC system first The separate Port Polska high-speed railway programme has already selected 2×25 kV AC for the new Warsaw–Łódź–Poznań/Wrocław network, designed for speeds of up to 350 km/h. Traction substation procurement is underway, with the Warsaw–Łódź section scheduled for opening in 2032 and the extensions towards Poznań and Wrocław in 2035. However, the existing conventional lines connecting to the high-speed network will retain 3 kV DC under the current Port Polska plans. Trains operating across both networks will therefore require multi-system traction equipment. PLK's new electrification programme raises a separate question: whether selected existing main lines could eventually be converted to AC, reducing the long-term coexistence of different power systems. What would conversion mean for railway operators? Changing an existing line from 3 kV DC to 25 kV AC would have direct consequences for passenger and freight operators. Locomotives designed exclusively for 3 kV DC could no longer operate electrically over converted sections. Operators would need multi-system locomotives, compatible replacement rolling stock or locomotive changes at system boundaries. For freight operators, however, higher-capacity AC electrification could offer operational advantages. Better power availability and reduced voltage drops could support high-powered locomotives and heavy freight trains, particularly where the existing traction power supply restricts performance. Neighbouring Slovakia provides a recent example. On 28 September 2026, the Žilina railway junction changed from 3 kV DC to 25 kV AC, moving the electrical system boundaries away from the station and ending regular operation by DC-only locomotives through the junction. Czechia is also progressing with its programme to replace 3 kV DC electrification with 25 kV AC. Poland's approach remains at an earlier stage. …