不定式作定语：用来修饰被序数词限定的中心词，且与中心词为逻辑上的被动关系。譬如：
1. This was the moment when Neil Armstrong became the first man to stepon the Moon 就在此刻，内尔·阿姆斯特朗成了第一登上月球的人。《柯林斯高阶英汉双解学习词典》
2. I'd like to fight him because he's undefeated and I want to be thefirst man to beat him 我想和他对阵，由于他从没输过，而我想成为打败他的第一人。《柯林斯高阶英汉双解学习词典》
3. He was the first man to navigate the Atlanticby air. 他是第一个飞越大西洋的人。
那为何这个句子用的是目前分词呢？
The first bullet train linking Beijingand the Xiongan New Area departed from the Beijing South Railway Station onThursday.
我看到就填了 to link，一看答案是 linking，感觉非常奇怪。
你之所以选择错误，是由于你没仔细领会不定式作定语时的意义，仅仅看到结构相似就作了选择，忽视了句子表达的逻辑意义。以下三个不定式作定语的句子，不定式其实表示的都是完成体的意义，若改为定语从句，则应为完成时态。比如
1 This was the moment when Neil Armstrong became the first man to step on the Moon
= ... became the first man who had ever stepped on the Moon.
2 Id like to fight him because hes undefeated and I want to be the first man to beat him
= ... to be the first man who has/will have ever beated him.
3 He was the first man to navigate the Atlantic by air.
= ... the first man who had ever navigated the Atlantic by air.
即以上三句中的不定式都是事件意义，表示曾做过某事件的第一人。不定式及定语从句的先行词为序数词+名词。
而你的句子中，作定语的link却不是事件意义，而是习惯意义，若改为定语从句作定语，则从句用一般体，而不可以用完成体。比如：
The first bullet train linking Beijing and the Xiongan New Area departed from the Beijing South Railway Station on Thursday.
= The first bullet train that links Beijing and the Xiongan New Area...
目前分词和定语从句的先行词都只不过名词bullet train，不包含序数词。
序数词+名词+to do不定式只能表示事件意义，不可以表示习惯意义，所以需要用目前分词作定语，表示习惯意义。
