How do you ride across the whit rails with your bike in Pokemon sapphire?

You can't ride across the white rails with your bike in Pokemon Sapphire. The white rails are decorative elements and not meant to be ridden on.

However, there are certain areas in the game where you can use your bike to travel on rails. For example:

* Cycling Road: This road, located in the northern part of the Hoenn region, has rails that you can use to travel faster.

* Battle Frontier: You can find rails in some of the challenges at the Battle Frontier.

It's important to note that you cannot simply ride your bike on any rail in the game. You need to be in a specific area where the rails are designed for cycling.