A further misconduct hearing for sacked chief constable Sean Price relating to 11 other gross misconduct allegations will no longer take place, Cleveland Police Authority said.
However, Price remains on police bail pending further inquiries in relation to Operation Sacristy, a wide-ranging inquiry into alleged police misconduct.
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The incident is not believed to be terror-related, British Transport Police said.
The royal couple have been in the US to mark the 300th anniversary of the foundings of New Orleans and San Antonio.
A small number of people were treated at Southgate Tube station, British Transport Police said.