翻译：正当我买票的那个时候（过去的某时）,火车启动了.、
just as在这的意思,应该分开理解.just是正当,正好.as是当...什么的时候.
其它意思：
Just as I expected.
和我预料的一样.
Just as I thought.
正如我所料到的.
Just as you say.
随你的便.
claim as due or just.
要求收回公正地或适当地宣称.
Come just as you are.
你就这样来（不做特别准备）.
Do just as you choose.
你愿意怎么办就怎么办.
Just the same as always.
就跟往常一样.
Just as you like.
随便你吧.
Just as it used to be.
没有一丝一毫的改变.
Just leave it as it is.
就让它去.
The moon goes around the earth just as the earth goes around the sun.（月亮绕着地球转就好像地球绕着太阳转）
其中的just as是就好像的意思.
我也喜欢英语,希望多交流.