Dark is good. One thing I've done in a recent project is just alter the color slightly a couple of times. It only takes a little color change randomly dispersed among the ties to give the impression of variation.
What actually bothers me about these isn't the tie color but the rails. If it is a totally abandoned rail then it will be rusty all over - similar to what you have here but redder. If it's an active rail line it will have a shiny metal top surface while the sides will be duller and a little rusty similar to what you have here.  The rail car metal wheels keep the top surface burnished bright. That can be created using a mixed material and altitude.