You may stay at Hangzhou city first, and then visit Zhangjiagang.

Zhangjiagang is under the jurisdiction of Suzhou city and no train station is available there. But you could take a coach to Zhangjiagang from Suzhou city, which only takes about 40 mins with ticket price around CNY30. From Hangzhou to Suzhou, many trains are operating daily and the fastest ones only cost about 2 hours. You could refer to Hangzhou Train(https://www.travelchinaguide.com/china-trains/hangzhou-schedule.htm) for the details.