None of the trains in the traincraft 1.6.4 version will slow down when pressing the s key I think it might be conflicting with other mods but I've tried to change key binds and nothing appears to be working I'm playing with the tekkit main pack and Flan's.

1.6.4 isn't supported by us. We can't provide any help for it. The version should work. Make sure it's not conflicting with anything else.