It's been aaaages since I did a #railwaysExplained, so let's have a look at why the @Eurostar class 373 has so many power settings...

1/ https://twitter.com/EurostarJustinp/status/1290751089273450498

we have to start with what the symbols mean. @dasy2k1 helpfully explained:

~ is AC
= is DC
BR = British rail
HS = high Speed
ET = eurotunnel
F = France
B = Belgium
GV = grande vitesse

2/

Let's work right to left:

BR~ is British Rail 25kV AC overhead line, for the domestic network

So that will mean AC pantograph raised, DC shoegear retracted

3/

HS is for High Speed 25 kV AC OLE on HS1 (UK).

So how is that different to the BR AC setting? After all, the voltage is the same.

4/

The difference is due to the mechanical characteristics of the OLE. The BR setting matches the pantograph pressure (90N) to the (then current) standard of 11kN tension on the contact wire

5/

The CW tension on HS1 is 20kN so it needs a different pan pressure - I'm assuming the pressure is higher, does anyone know?

ET is for 25kV AC OLE in the Tunnel - a different tension again

6/

BR = is for 750V DC third rail. We don't talk about that one in polite company

GV~ is for the high speed 25kV AC OLE on the French side. This is the same tension as HS1 equipment, so not sure what is different on that one

7/

F~ is for 25kV AC on conventional French lines - again, this will have a different tension so a different pressure is applied.

8/

B= is for 3kV DC Belgian Equipment - because the voltage is lower there, current will be correspondingly higher, and so a different pantograph with more/wider contact strips is needed to provide the required surface area. Otherwise the pan will overheat

9/

I'm assuming that further round the dial, out of sight, there is also F= which would be for 1500V DC OLE in France. This will also use the DC pantograph.

10/