The station was, in the unconscious mind of Harry, clean and empty except for something that looked like a small, naked child, curled on the ground, with skin that was raw and rough, as though it had been flayed, which lay under a seat. Dumbledore then revealed to Harry that the form represented a small part of Lord Voldemort, destroyed when he "killed" Harry and thus annihiliated the part of his soul which he had unwittingly implanted in him.
-King's Cross Station, Harry Potter Wiki
我闲下来搜了一下哈利波特维基,相关的话是上面这样。这样看来HP Wiki的编者就应该是认为是灵魂碎片了,但是我没有找到任何更直接的依据。罗琳在pottermore中提到过国王十字车站,不过我还没有玩到这一章。之前有吧友做过的翻译是在这里:
http://tieba.baidu.com/p/1872621518?see_lz=1可惜也没有哈利和邓布利多这一段相关的部分。
总结说来,认为是灵魂碎片或许是一个比较主流的看法,不过我还是没有找到更直接的依据,具体可能还是要问问罗琳本人。抱歉没能帮上什么忙。