Yes — a diploma in the relevant engineering discipline is one of the accepted qualifications for RRB Junior Engineer posts.
The diploma must be in the discipline matching the post, like mechanical, electrical, civil or electronics.
Candidates with a B.E. or B.Tech in the relevant branch are also eligible.
The qualification must be complete by the cut-off date given in the official notification.
Always confirm the post-wise qualification table in the current RRB JE advertisement.