The LEGO RC/9v train switches seem to be very easy to break: just
accidentally step on the yellow lever and soon one of the clips holding the lever will
snap:
This is an easy way to fix such a broken switch: you will need a 1 x 6 plate (3666)
and a 1 x 2 x 1 panel (4865). Both can be easily ordered on Bricklink. The
correct colour is "dark bluish gray". The plate will be used as is, the panel
will have a hole drilled into it:
To drill the hole, use a 2 mm bit.
You can easily mark the correct position for the hole by putting the plate and
panel in place and hand-scratching it with the drilling bit through the
other non-broken clip.