You need to build the Academy of Industry 🏛️ to upgrade train lines. Open the Academy to see the list of factories, train lines and islands you can improve. These are the attributes you can upgrade using ingots:
- The number of experience points for filling railcars;
- The time a train takes to arrive ⏰

You can't focus on upgrading just one attribute. Instead, you have to gradually improve all the different attributes to unlock the upgrades available at the Academy of Industry.

Different upgrade levels require different numbers of ingots, which are produced at the Foundry.