I will add to the comments above by Martinvk.
Because the DLS is free to all Trainz users there is no economic incentive to copy, alter and re-post the work of others - unlike movies and music. This also removes the likelihood of legal action against violators because, as has been discussed elsewhere in these forums, such action would be very expensive for the copyright holder and the violators themselves are extremely unlikely to be wealthy.
The best we can do is to be vigilant. If some-one notices a new asset that they suspect is an unauthorised copy then they should inform the original creator who is the only one who can confirm (or not) if that suspicion is a fact . If confirmed, and if the original creator is prepared to take the matter further, then the usual N3V reporting process can be used to remove the item(s). In the meantime a warning can be placed in these forums.
This, of course, only applies to the DLS. Third party web sites may have different procedures or none at all. If the web site belongs to a violator who is unwilling to remove the asset then there may be very little that can be done, beyond issuing warnings in these forums.
I agree with the comments that once you post something on the Internet it is out of your control. If someone wishes to copy, alter and re-post any of my work then I would hope for, at the best, an acknowledgement. But I will not lose any sleep if I don't get that.