As said, this is not an area that I know much about. However, I think that the original acts were to allow the railway company the powers to build a line: they did not confer any obligation to operate trains
I've had a re-read of the original acts of Parliament regarding my old branch line. There is absolutely no mention of any obligation for the L&Y (Lancashire and Yorkshire railway company) to operate trains in it, only to build the line.
However, I think that many Acts included time limits for construction
That's certainly true of my old branch line.
The original act 18 May 1866 (Blackburn to Padiham section) had a time limit of four years. An act 15 July 1867 granted the Padiham to Rose Grove section and extended the Blackburn to Padiham section to 1 August 1871.
As I've mentioned many times before, this line was a challenging one to build...
The L&Y had to apply for extensions of time which they did in 1869, 1871, 1873, and 1875 taking the final time to build it up to 1 August 1876.
Once you had an act approved it was up to you to either build it, or apply for an act to abandon it, however the L&Y tried a third option: In 1872 they applied to Parliament to suspend building the line indefinitely, Parliament said No to that one so the L&Y simply had to carry on building it (or abandon it).
How strict Parliament was regarding completion dates is anyone's guess. The Padiham - Rose Grove section was within the time limit, however the Blackburn - Padiham section wasn't. Newspaper reports from August / September 1876 report the L&Y stating the full line would be opened by the end of the year (Dec 1876) which is beyond the time limit. Even worse than that, January 1877 an embankment collapsed after weeks of heavy rain, it took six more months to repair and the first train (freight) ran 1 June 1877 - well out of time!
I can understand an embankment being washed away, but not the time from August to the end of 1876? Perhaps, if you were seen to be making all reasonable attempts to complete a line's construction then that was good enough?