This town was modernised. These towns were modernised. Watch out for single was or plural were.

This town had been modernized since the Mayer took the office. In time sequence: The first incident: This town was modernized. (had completed the modernization) The second incident: the Mayer took the office. If you want to stress the time sequence, you may use "past perfect" to feature former and the latter.

"This town was modernized." - This sentence is in the past tense. It tells us that the modernization took place at some specific point in the past. Simple past tense allows the speaker to add information about the specific time when something occurred. Speaker A: "When was the town modernized?" Speaker B: "The town was modernized during the period of 1955 to 2010." "This town had been modernized." - This sentence is in the past-perfect tense. It tells us that the modernization occurred before some other event which is also in the past. When using perfect tenses, the speaker cannot add time information about when an event occurred. Perfect tenses only convey information about the relative order of events, for example "The town had been modernized before I was born.". The sentence you provided is incorrect because it attempts to add information about a specific time (1955-2010) to a perfect tense which isn't possible in English. In cases where you need to specify a particular point in time you should use the past tense.