Be able to upgrade classes and trains: Instead of just using the classes & trains and calling it a day, why not upgrade them further?

Lvl 0: The original stage of the classes & trains
Lvl 1: 10% buff on all classes & trains. This upgrade will cost ~5% more than the class/train is originally worth.
Lvl 2: 25% buff on all classes & trains. this upgrade will cost ~20% more than the class/train is worth.
Lvl 3: 40% buff on all classes & trains, also the maximum upgrade for the classes that cost ≤50 bonds, and the maximum upgrade for the trains that cost ≤300 bonds. This upgrade will cost ~35% more than the class/train is originally worth.
Lvl 4: 55% buff on all classes and trains. This upgrade will cost ~50% more than the class/train is worth.
Lvl 5: 70% buff on all classes & trains, also the maximum upgrade for the classes that cost >50 bonds, and the maximum upgrade for the trains that cost >300 bonds. This upgrade will cost ~65% more than the class/train is worth.

(PS: You guys can decide on what the buffs are going to be because I don't want to give five upgrades to all 20+ classes.)
(PPS: Give credit to the ones who suggested the features by putting a name next do the featured structure or a “Update Logbook” (if you ever add those).)