[votan]

From what you have posted I don't see a mistake either.
Try this:

<Bindings>
  <Layer name="SI_KEYBINDINGS_CATEGORY_GENERAL">
    <Category name="Fast Ride">
      <Action name="FASTRIDE_SWITCH">
        <Down>SLASH_COMMANDS["/fastride.key"]()</Down>
      </Action>
    </Category>
  </Layer>
</Bindings>