Alternating current (AC) electrification system
The first experimental railway line electrified using AC 20 kV was Ozherelye - Mihailov - Pavelets in 1955-1956. After successful testing the voltage was increased up to 25 kV.
First AC electric-multiple unit trains began to operate in 1957 on railway line Gorkiy - Pavelets 47 km long.
The AC 25 kV system became widely used since 1959 when there was need to electrify large area and there were no DC electrified railway lines.
Now almost all new-electrified lines are using AC 25 kv 50 Hz power system.
To connect lines with different electrification systems together special junction stations are built. These stations as a rule have switcheable catenary allowing electric locomotives of different current types to exchange.
There are some electric locomotives series that are able to operate using DC and AC: VL82, VL82M, EP10, EP20. There're also some electric-multiple unit trains series that are able to operate using DC and AC: HRCS2 (built by Hyundai), EJ675 (Škoda Vagonka), EVS2 (Velaro RUS), ES1 (Desiro Rus) and Sm6 (Allegro)