There are still treated wood products available. It is true there were some big changes in what chemicals could be used, but you can still get treated lumber for outdoor projects. Bear in mind no wood will last forever when it is exposed to constant ground contact. If these are just upright posts, treated wood would not be necessary, just paint will provide protection. If these are supporting the porch floor, then what about concrete piers or blocks? I am having a hard time understanding why you would want to use creosoted railroad ties for the pillars that support the porch roof.