If you are in 12th class you can take conventional engineering colleges and complete your degree in any of the branch mentioned above. Railways conduct RRB exams for each branch separately. You need to prepare hard to get through this exams. so choose the branch on the basis of what you like and what you can do. Railway used to take exams after 12th for engineering but this year it's cancelled.

Dear Tarun

Tarun inorder to join the team railway there is not any type of branch specifications will be there only as per the requirement they want only the UG category that's all and if you wish to join the railway then after the completion of your graduation there will be n number of exams of Indian railways for various post so as per your convenience you can fill up those forms as well there will be a specific post in an Indian railways which is also called as Section Engineer & if you wish to join that you can go with any of the core branch in engineering like Electrical, Mechanical etc.

All the best