Just think about it, We spend a month upgrading and upgrading our trains and when the round of over everything is lost.
If, like me, you love seeing that you make some progress in a game, I can encourage you to look closely at the carrier achievements.
I personally see the game like that now : you start rounds after rounds, and the goal is to unlock one or two of those specific achievements.
And my carrier engine is then the reflection of my progress.
Otherwise, there are also the skins that stay unlocked, I always like to use the vouchers I received on skins of trains that were particularly of use to me throughout a particular server.