What indicates that you are approaching a railroad crossing?

A yellow circular sign with an "X" inside is specifically designed to indicate that you are approaching a railroad crossing. This type of sign, also known as a crossbuck, serves as a warning to drivers that they should be alert for train crossings and prepare to stop if necessary. The "X" symbol is universally recognized as a sign for railway crossings, helping to alert drivers to the potential danger.

In contrast, other options like a stop sign indicate that drivers must come to a complete halt but do not specifically signal the presence of a railroad crossing. The green light is typically associated with traffic signals indicating go, and a warning bell is often associated with the crossing itself but may not serve as a primary indicator for approaching a crossing from a distance. Therefore, the yellow circular sign with an "X" is the most effective and recognized warning for an upcoming railroad crossing.