Board Thread:Questions and Answers/@comment-80.56.120.109-20140116153257/@comment-5596770-20140116201630

For additional detail..

TrainStation starts in the Steam Era, meaning you can only use Steam Locomotives.

At level 25, you may purchase Dieselisation to enter into the Diesel Era, which allows you to also field Diesel Locomotives. Purchasing Dieselisation does not remove the ability to use Steam Locomotives, it only adds the option to also field Diesel Locomotives.

At level 40, you may purchase Electrification to enter into the Electric Era, which allows you to also field Electric Locomotives. Purchasing Electrification does not remove the ability to use Steam Locomotives (or Diesel Locomotives, if you've purchased Dieselisation), it only adds the option to also field Electric Locomotives.

At level 120, you may purchase Maglev Era, which allows you to also field Maglev Locomotives. Again, this upgrade does not remove the ability to field any locomotive types, it only adds the option to also field Maglev Locomotives.