Dave Liquorice wrote:
(*) Invertor would normally be a device to convert battery DC to AC
(1 or 3 phase) rather than a convertor taking a single phase supply
and creating a 3 phase one. I assume this device is the latter.

Inverter is the common term for a device that controls a multiphase
synchronous motor. You feed it with DC at whatever voltage you have to hand
and the inverter, typically by means of an H-bridge (as many legs as there
are motor phases, usually three or more), generates variable-voltage
variable-frequency (VVVF) AC for a synchronous motor.

On trains it's common to transform single-phase 25kV down and rectify to say
750V DC, before feeding into the inverter. The controls on the inverter
adjust the VV and VF to control speed, torque, regen and account for things
like wheelslip. Inverters have replaced older controls such as tap
changers.

I assume here the inverter is running off rectified mains to provide the DC.
You can make AC to AC converters without a DC link, but they're only
competitive in the MW range:
https://en.wikipedia.org/wiki/Cycloconverter

Theo