The two firms will enter the competition for the West Coast Partnership franchise, which will cover services on the existing West Coast route from 2019 and initial HS2 trains between London and the West Midlands from 2026.
They will also bid together for the East Midlands franchise starting in July next year.
FirstGroup, former operator of the ScotRail franchise, runs Great Western Railway, TransPennine Express and Hull Trains.
Earlier this month, Italy’s Trenitalia took a significant step to becoming the latest European state-owned operator to run services in the UK after striking a £70 million deal with National Express for its c2c rail operation.
First Rail managing director Steve Montgomery said the company has “a strong track record in delivering continued modernisation and investment” in rail services.
He said: “Both ourselves and Trenitalia have extensive expertise of running commuter, regional and long-distance services such as those that make up the East Midlands franchise.
“Their knowledge will add further depth and understanding as we develop our proposals to deliver passenger benefits that take full advantage of the significant investment that government and the industry are making in the region’s rail infrastructure. Our proposals will keep people moving and communities prospering.”
Montgomery said Trenitalia has “transformed the customer experience” on Italy’s railways through the high-speed Frecciarossa routes, and both companies would deliver “the best solutions” for HS2 while “reducing crowding on the existing network”.
Trenitalia chief executive Barbara Morgante said: “We believe that the experience, competence, innovation and skills of our two companies, developed in international markets, represents a perfect mix to vie for these challenging bids.”