It's not the case that RMT members at GTR didn't vote for strike action, it's that there appears to have been a mass outbreak of apathy, so the thresholds weren't met.
Ballot Result said:
Number of individuals entitled to vote in the ballot: 2327
Number of votes cast: 1198 (51.5%)
Number of individuals answering "Yes": 926 (39.8% of those eligible to vote)
Number of individuals answering "No": 267 (11.5% of those eligible to vote)
Number of spoiled or otherwise invalid voting papers returned: 5 (0.2% of those eligible to vote)
For a ballot to succeed, there
MUST be a turnout of at least 50%
AND at least 40% of those eligible to vote have to vote Yes.
To put it another way, if those 5 "spoiled" ballots had been unequivocal "Yes" votes, it would have succeeded.