//NJ, The RMT ballot of underground drivers voted over 90% for a strike. The picket line is not to deter the handful of drivers who did not want to strike, it is a public show of their grievance.//
The public - especially those who need to use trains - is well aware of their grievance. Their leaders are interviewed on prime time TV and radio. Standing with placards around a brazier outside a works entrance does nothing to enhance that. Here's the Local Governments Association's take on it:
https://www.local.gov.uk/our-support/workforce-and-hr-support/employment-relations/employment-law-topics-and-e-guides-7
"Picketing occurs when a group of people gathers outside a workplace to try and persuade others, such as non-strikers, substitute workers or suppliers, to take some form of industrial action."
And an employment lawyer's take:
"The purpose of the picket is to peacefully encourage non-striking employees and members of the public to oppose a lock-out or to support strikers involved in a protected strike."
Picketing is an antiquated practice which serves no useful legitimate purpose today. Far from being a protected activity, it should be outlawed.