Disadvantages
A 25 kV AC system uses only one phase of the normal three-phase electric power supply. This results in an imbalance on the three-phase supply which may affect other customers. This can be overcome by installing static VAR compensators or reducing the traction load when the imbalance becomes unacceptable. The system is not insulated from the distribution network, like other systems. Older locomotives and the recuperating electrodynamic brakes on newer locomotives create electrical noise. It is not necessarily practical to filter this noise from the electricity distribution network, and this has led some countries to prohibit the use of recuperating brakes.
The high voltage leads to a requirement for a slightly higher clearance in tunnels and under overbridges.
To avoid short circuits, the high voltage must be protected from moisture. Weather events, such as "the wrong type of snow", have caused failures in the past. An example occurred in December 2009, when four Eurostar trains broke down inside the Channel Tunnel.
Read more about this topic: 25 K V AC Railway Electrification