It is argued that high speed rail delvers many economics benefits: HS2 is a controversial project which if build would join London, Birmingham, Manchester and Leeds and would cost roughly £42 billion. The real economics argument in favour of HS2 is that it could transform the economies of the midlands and the North, it was also designed to combat infrastructure for example the UK being 27th in the World Economics's Forum's League table for quality of infrastructure. The main argument against applies on the principle of "if we build it they will come" but would this really happen? Some people argue Canary Wharf is an example of this in the UK. This issue is still mutely under discussion an interesting report is found here.
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/365065/S_A_1_Economic_case_0.pdf