At the earlier stages, the vehicles were controlled by hard track lines without any network operation.

早期的城市轨道交通车辆一般由硬线控制，没有网络或者网络不参与控制。

youdao

It's hard to keep track of the children's comings and goings.

这些孩子来来往往，很难跟得上他们的行踪。

《牛津词典》

youdao

What did you learn about how easy or hard it is to keep track of what food intake is?

你从记录食物摄入过程中,学到了什么,记录饮食是难是易

耶鲁公开课 - 关于食物的心理学、生物学和政治学课程节选

It's just hard to keep track of them at first but they all generally mean the same thing, provide additional input to this program called GCC by just specifying it on the command line.

刚开始是很难理解这些词的,但其实都是一个意思，所以，只要在命令行上为这个程序,再添加一个名曰GCC的输入就OK啦。

哈佛公开课 - 计算机科学课程节选

The 10-place grid penalty in China was very difficult as it's a hard track to get close to people and overtake so, in many ways, I knew that was going to hurt us.

BBC: Mark Webber column

It is hard to track statistics for a part of the economy so informal and atomised.

ECONOMIST: South Africa

The global nature of the organized crime syndicates involved makes them hard to track down and prosecute.

WSJ: Soccer Match-Fixing Probe Uncovers Suspicious Games