Seven catering vehicles used by Abellio Greater Anglia will be converted with 54 standard class seats fitted in place of the current 24 first class.
Additionally, 29 standard class Mk 3s will be fitted with an additional four seats as part of the refurbishment scheme that starts next month.
The work will be carried out by Vossloh Kiepe at Norwich Crown Point. Fifteen additional jobs will be created for the project.
The overall impact means that rather than over 1,600 extra standard seats being provided each week day, as was announced earlier this year, there will now be over 2,500 extra standard seats each weekday - equivalent to over 12,500 seats a week, over 50,000 seats a month or well over 600,000 seats a year.