A Four-Quadrant Gate system is a system in which a railroad at-grade crossing has four crossing gates protecting it instead of just two.
In addition to the two crossing gates protecting the crossing from traffic approaching the crossing, or the entrance gates, there are two additional crossing gates on the opposite sides of the roadway that close off the lanes of traffic exiting the railroad crossing, or exit gates. The two additional gates are there to prevent drivers from driving around the regular crossing gates. When all four gates are lowered, the crossing is completely closed off to vehicular traffic until the gates raise back up to their home position.
How some 4-quad gate crossings work is when there is no train immediately approaching, all four crossing gates are up and the signals are inactive. But when a train is on the way, the signals in place will start up (lights flashing and bells ringing.) Then the entrance gates will start to go down but the exit gates will remain up a couple to a few seconds more before lowering. Once all the gates are down, as mentioned, the crossing is completely closed off to vehicular traffic.
A Four-Quad gate system is not exactly a new technology, four-quad gates have been used in the early 20th century when gates were black and white. They also took place at manually operated crossings presided over by a watchman who would lower the crossing gates.
Most of the few railroad crossings on Amtrak's Northeast Corridor are 4-quad gate crossings.
They are also used on other lines used by Amtrak, including in Illinois, one such line is used by Amtrak's Texas Eagle.
Trinity Railway Express (TRE) (Dallas-Fort Worth) uses 4-quadrant gate crossings.
4-Quad gates crossings are used in the United States, the United Kingdom and some European countries including the Czech Republic