Appearently it's not clear for all users that there is copyright on submitted graphics (necessary code included). Copyright belongs to the artist who made the graphics. Always read the copyright notice included in submitted graphics or, if a copyright notice is missing, contact the artists before distributing a (modified) version of the graphics.
Most artists allow modification of their graphics for personal use. That means that you can modify graphics for use on your own computer but you cannot distribute the modified graphics without permission.
Many graphics for Locomotion are based on graphics that were initially drawn for MicroSoft Train Simulator (MSTS). The artists that made those models are often not registered on this forum. That does not mean that permission is automatically granted. Whoever the artist is, forum member or not, you should always ask permission to modify the original graphics.
The copyright of other materials is also covered by the copyright disclaimer in the forum rules:
All original "Transport Tycoon", "Transport Tycoon Deluxe" and "Chris Sawyer's Locomotion" graphics, sounds and music here are the intellectual property of Chris Sawyer and/or Atari, Inc. Copyright © Chris Sawyer 1994-1996, 2004. All rights reserved. The use of these graphics on The Transport Tycoon Forums is believed to constitute "fair use".
If anyone has any complaints regarding copyright violations, please follow the process in Rule 4(e) to notify an administrator about your concerns.
Consequences of breaking the rules
If you are identified as a person who intentionally distributed graphics or any other copyrighted material without permission from the artists you will receive an official warning.