Productivity doesn't just happen. Either you are getting more efficient with the same work, or working more. The benefit
gets captured by the people creating it when labor markets are efficient. And labor markets tend to be super efficient, with some corner case exceptions.
The 'rank and file' won't get more if their labor is not generating the gains. The labor market is efficient, people are generally getting paid what their skills are worth. However, if a laborer works twice as much as his peers, he will generally get paid twice as much as employers will recognize his value and compete for his services.
It is rare that collective bargaining is a net positive. Very unusual cases where the employers have highly concentrated labor market power and ability to use anti-competitive strategies. For instance the NFL, where a small group of employers (32 teams) can work together to cap the amount of revenue devoted to labor (players). In this case the labor market is distorted by excess market power in favor of employers, and it makes sense to allow a union to negotiate collectively to offset the market power of the employers.