How to add constraint in Double track Ballast?

Difficult to give a simple answer. You need to define how the subballast and ballast should transition from a double to a single. So you must set a double track template where one track will be controlled by its alignment using a point control. Then in the template you must define and test what is happening when one track is moving over the other one. Usually you will have a single slope for the subballast to support the second track until a defined distance between the 2 centerlines.

Dear sir,

Thanks for your response, actually, I faced the problem when adding the constraint in double track template, I don't know how to add constraint properly like this template (you may see in template folder MPD  'MPD_Double_Track'), after transitioning between 'MPD_Single_Track' & 'MPD_Double_Track' template it is not working properly when one track is moving over the other one.

this template should not transition to single. In the corridor, for the double track template, you should control points to move a track over the other one "double to single"

Thank you for your suggestion.

Thank you for your suggestion.