Page "Transformer" Paragraph 62

One example of state-of-the-art design is transformers used for electric multiple unit high speed trains, particularly those required to operate across the borders of countries using different electrical standards.

The position of such transformers is restricted to being hung below the passenger compartment.

They have to function at different frequencies ( down to 16. 7 Hz ) and voltages ( up to 25 kV ) whilst handling the enhanced power requirements needed for operating the trains at high speed.