Hi,
The By instance ID checkbox should do the trick. Let me add, that the RailClone plug-in provides a powerful set of tools to control the UVW mapping and Material IDs of segments. Not only is RailClone able to Randomise material IDs and adjust the position, rotation, and scale of existing UVW coordinates, but it is also possible to generate new box-mapped UVWs automatically that follow the source path. This is ideal for situations where you need the texture to bridge multiple adjacent segments and create the illusion of a seamless unified surface.
Best regards,