In this Trigger Apex,we have two custom sobjects Train__c and Passanger__c. At in Train__c there are two fields Passanger_Old_Phone_Number__c and Passanger_New_Phone_Number__c. At in Passanger__c there are one field Phone_Number__c. If the Phone number in Phone_Number__c was changed by Passanger in Passanger__c then old phone number  can be store in Passanger_Old_Phone_Number__c and new phone number can be store in Passanger_New_Phone_Number__c.
There are Lookup Relationship between Passanger__c to Train__c

We want functionality in Trigger Apex
Thank you !