Pretty much confirmed that Ford will only play Indiana Jones in the framing sequence.
https://www.dailymail.co.uk/tvshowbiz/article-9653211/Train-transformed-Nazi-carrier-engine-ahead-filming-new-Indiana-Jones-movie.html
If they are building a Nazi train, then obviously the film takes place in the 1930s-40s, so unless the Nazi train is just in it for a brief flashback, or Old Indy time-travels back to WW2, or they use makeup or CGI to make Ford look young, another actor is going to be playing a 40-something Indiana Jones in this.
If we match Ford's age, his scenes would have to take place in the 1960s or 70s, and there were no Nazi trains operating at that time, for those with a weak grasp of history.