By the end of the century, almost enough rail track to encircle the world covered this small island, altering the nature of travel for ever and contributing to the industrial revolution.
This sentence is taken from a test question, I cannot understand why after the comma in the beginning of sentence there is not any verb. I mean it needs something like this. (There has been built almost enough rail track to encircle....)
You're not parsing the sentence correctly. What you have bolded is only part of the sentence, it's not a subclause.
"almost enough rail track to encircle" is not correctly delimited.
Notice the similar structure with an example sentence:
[The table cloth] covered [the table].
[Almost enough rail track to encircle the world] covered [this small island].
"Covered" is the verb you're looking for. Everything that comes before it is a noun phrase, which is being used (as a whole) as the subject of the sentence.
