If you fill the low side of the area, only bring it up to level with the existing dirt line at the tree! Anything above that may cause damage to the trunk via insects and more likely rot.
The root system will over time work its way back to the surface, but could/may also sustain damage due to oxygen starvation if the fill is compacted heavily while your building the railway.
You'll see at a minimum a weaker tree on the raised bed side for at least 2 to 3 years, meaning the canopy will be diminished, and possibly paler in color. With the tree in stress it will also be more suseptible to diseases and insects during this time so watch for that.
Good luck with whatever you do...
Mark
The root system will over time work its way back to the surface, but could/may also sustain damage due to oxygen starvation if the fill is compacted heavily while your building the railway.
You'll see at a minimum a weaker tree on the raised bed side for at least 2 to 3 years, meaning the canopy will be diminished, and possibly paler in color. With the tree in stress it will also be more suseptible to diseases and insects during this time so watch for that.
Good luck with whatever you do...
Mark