Here is an RCX-driven level crossing used within my town,
together with some videos showing it in action.
 
Both bars are operated by the same motor, hanging (and hiding)
under the train tracks. The tracks have a nice feature (usually
referred to as a "hole") used to transfer the movement with belts
from below the tracks to the gear boxes. Since the weight on the
bars is definitely not distributed evenly, the imperfection of the
gears makes the bars shake/vibrate when going down. In order to
eliminate this phenomenon, paper-padded gear boxes are used (until
a better solution is developed, e.g. with micromotors).

Two sensors measure the train's speed and length, used to
estimate the train's time of arrival to the level crossing,
as well as the moment when it is supposed to be gone.
The traffic lights and bars act accordingly.

The videos here show the pre-production, one sensor version (with
the train length preprogrammed), but the final appearance is the same.
  * v_crossing01.avi: Traffic light & 1 bar (880KB, XviD)
  * v_crossing02.avi: Mechanism uncovered, 2 bars (600KB, XviD)
  * v_crossing03.avi: Bird view of the town (1.93MB, XviD)

Any questions, suggestions and comments are welcome :)

Matija
puma (at) iskon.hr