I feel that upgrading your trains is totally not worth it
im making way more money with my 1910 locomotives that the one I unlocked in the 30s and 40s. i expanded through half of the map already, my main money making lanes are transport that goes not higher than 80km/h and carry around 80-90 ppl.
Getting a better train would skyrocket my cost from around 100k/year to 1M-2M/year while making not much more money. So I'll be earning less money per lane and upgrading the cities at the same rate.
my unique problem is that they will be gone as year passes and im forced to get newer trains, so im forced to get way less money while everything gets precier.
Edit: I feel that a solution for this would be a better balance of maintenance cost, right now they're too expensive to mantain for the value they give compared to other trains.
It would be a better option to have a higher front cost than buying 2 or 3 teains but having more or less 2 trains of lower tech