I am going to put 4 main lines in and didnt know how far apart they need to be apart from each other on curves and strait aways
1-1/2 inch is safe. Its the NTRAK standard. Scales to 20 feet so is a little wide for most prototype practice. Since our curves are much sharper than prototype overhang can be a real problem.
If you want to consider closer spacing, Heres the link to the National Model Railroad Assoc. Standards One of them covers track spacing