Level Crossings, known as the Railroad Crossings in the US are the places where the road and railway meet and intersect. They consist of two gates on either side of the crossings. Usually, the gates need to be opened and closed by a person, but in more recent episodes, they have been operated by nearby signal boxes.
In the Railway Series, the television series and in real life when engines approach a level crossing, they always have first priority and the road vehicles have to wait. However in the CGI series, the road vehicles have first priority if they reach the crossing before the engine does, meaning the engine has to unrealistically stop.
While the gates are meant to stop traffic, there have been some occasions when engines and trucks cannot stop and wind up crashing through them. Some examples of these include Henry when he was running out of control along the line by Old Bailey's station, Toad when he was running loose with trucks, and Gordon when he had a banner stuck across his smokebox.