British Transport Police (BTP) said there were 150 fewer incidents recorded in 2014/15, but sex attacks rose by 16% from 19 to 22.
Overall, 1,172 crimes were reported - down on 1,322 the previous year.
BTP said crime in Wales remained low, but there was still much for the force to do.
Supt Andy Morgan, sub divisional commander for Wales, said sex crime figures in the region were the lowest of any of BTP's eight areas.
He said although it was concerning so many people were affected by this type of crime, the force had expected to see a rise after launching its Report It To Stop It campaign, which encourages victims to come forward.
The force said there were 42 fewer theft offences last year and instances where cables were stolen were down from 102 last year to 53 this year.
As a result, the number of travel minutes lost to the offence is down by 87% nationally since 2008/09.
Violent offences also decreased in Wales, at a time when there was an 8% rise across Britain.
The force said it had put more officers on night trains to deter problem behaviour and it was enhancing its CCTV coverage by trialling the use of body-worn cameras.
The report said there were 16 deaths on the tracks in Wales last year and BTP had introduced a hotline for staff to alert emergency services when staff are concerned about a person planning to harm themselves.

2 February 2016 Last updated at 15:09 GMT
School kids have been submitting their questions for the British astronaut.
Hundreds have travelled to the World Museum in Liverpool to see the live stream, and others are watching from classrooms around the country.
Tim is expected to be live at 2.30 to answer their questions.

Lawyers told a High Court judge on Friday that both sides had agreed to pause the legal fight for one month.
Sir Cliff is suing the BBC and South Yorkshire Police over coverage of the 2014 raid.
Lawyers for the singer previously said he had suffered "profound and long-lasting" damage from the incident.
Mr Justice Mann, who has been overseeing the preliminary hearings at the High Court in London, indicated that he would review matters in the near future.
Sir Cliff was investigated over historical sexual assault allegations, which he denied, and the case was dropped by the Crown Prosecution Service in June last year because of insufficient evidence.
The singer is seeking "very substantial" compensation because he says the coverage of the raid at his apartment in Sunningdale, Berkshire, invaded his privacy.
BBC editors have previously apologised for distress but have said they will "defend ourselves vigorously".
A spokeswoman said the BBC had reported Sir Cliff's "full denial of the allegations at every stage".
South Yorkshire Police have apologised "wholeheartedly for the additional anxiety caused" by the force's "initial handling of the media interest" in its investigation into the singer.
Crime on the railways fell by 11% in Wales last year, while the number of sexual offences reported rose, new figures show.
[NEXT_CONCEPT]
Tim Peake is preparing to talk to hundreds of school pupils in a live webcast from the International Space Station.
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Sir Cliff Richard and the BBC have agreed to try to reach a settlement over coverage of a police raid on the singer's home.