After the train had left the sation.火车开出车站后.

After the train had left the sation.火车开出车站后.
请问After是什么词性,在句中起什么作用?
Ater the train had left the sation,
the lady opened her handbag and took out her powder compact.
火车开出车站后，那位妇女打开了手提包，拿出了粉盒。
请问After是什么词性，这个句子是从句吗？什么从句？