Was the bill really passed in 1864? A quick search finds
the public notice of the bill's introduction in November 1846, which lists the parishes it goes through - pretty much in a straight line. Nothing about an extension there, but then that was before they had surveyed the initial route.
If you really want the full details, The National Archives says you should go to Truro:
Reference: QSPDR/17/1
Title: Plans and sections, Helston Railway. Penryn to Helston. With OS▸ 1 inch map
Date: 1846
Held by: Cornwall Record Office, not available at The National Archives
Those plans will be very interesting - as I found when I looked at some of the RGRR plans in the Berkshire RO.