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.
Crime on the railways fell by 11% in Wales last year, while the number of sexual offences reported rose, new figures show.