I don't think you want to use something that remains tacky forever, it will collect everything that comes in contact with it if you do. Dust, dirt, bugs, parts... anything. My first thought though would be to use rubber cement, although I'm not sure how long it will remain pliable. If it were me, I'd probably use white glue. It will harden, but you can soften it right back up and remove anything by soaking it with water. Right now I'm adjusting some track that has already been ballasted using white glue. I just spray it with water a wait for less than a minute before everything becomes loose again.