In the order to get job in the Railway department after B.Tech in Mechanical you need to appear in the entrance examination which is conducted by the UPSC. Following are the details of this entrance examination:

Eligibility:
The applicant should have completed engineering with minimum 60% marks from any recognized university of India.

Age limit:
You must have age between 21 years to 30 years.
Upper age relaxation is accepted if you belong to
SC / ST - 5 years.
Other BC - 3 years
Defense services personnel - 5 years
State of Jammu & Kashmir - 5 years.

Nationality:
You must be
An Indian
A subject of Nepal
A subject of Bhutan
A Tibetan refugee
Indian origin migrated to other country.


This is the list of various exams in railways for which you can apply after b.tech:

Indian Railway Service of Engineers (IRSE),
Indian Railway Service of Electrical Engineers (IRSEE),
Indian Railway Services of Signal Engineers (IRSSE),