If you already own an older version (like TS2017), the "core" engine usually updates to the latest version for free, but new routes typically require a separate purchase.
Generally, you must purchase the base game on Steam for around $29.99 . Train Simulator 2018 Download Free
If you are looking for a free experience, there are several official mobile games that use the "2018" branding: Train Simulator PRO 2018 - App Store If you already own an older version (like