Which type of pipe is typically found in older gas distribution systems that is no longer installed?

In older gas distribution systems, cast-iron pipes were commonly used due to their durability and strength. However, they are no longer installed because cast iron is prone to corrosion and degradation over time, especially in corrosive environments. This can lead to leaks and safety hazards, which is why modern standards have moved toward materials like polyethylene, which offers better resistance to corrosion, lower weight, and ease of installation. Steel pipes are still used in some applications but are typically protected with coatings to prevent rust and corrosion. PVC is generally not suitable for high-pressure gas applications, making cast iron distinct for its historical use in older systems that have since been phased out in favor of safer and more reliable materials.