Welcome to BPT Clicker
You are correct in using Distilled water and as Mr Ripley says the Minerals in tap water can collect and damage the the train. In fact I use distilled water in everything like coffee makers humidifiers, trains etc because of mineral deposits that can shorten the life of any appliance. One should always have at least a gallon on hand for such things at its really not that expensive, at least not in my neck of the woods.