Be careful! It's not always a truncation. I'm not sure to which "Kingston" you refer - there are many - but the borough/town in South West London of that name actually isn't a shortened version of "Kingstown" but rather the other way around - the name of Kingston has been recognised there for around 1,100 years and in Old English the word "tun" or "ton" meant a settlement or farmstead. Furthermore, it was a possesion of the king and several Kings are believed to have has their coronation there - thus "Kingston". Certainly, the word "town" developed from "ton", but it could well explain why a place like Jamestown uses "town" where as Kingston uses "ton".