Well all the Varanasi going trains are pretty much full, but if you want to travel in parts you can book your ticket upto Lucknow in 07305 UBL-GTNR special train on 28th April and from there travel in any Memu or other trains upto Varanasi.
Or you can take a risk book a waiting ticket in any DDU bound train.
Another risk taking option is booking a waiting ticket upto Prayagraj in SC-SFG special train, chances of clearing are pretty high in that train and also remains mostly punctual.